diff --git a/src/calendar/commemorations.ts b/src/calendar/commemorations.ts
index e043338..91e2bdf 100644
--- a/src/calendar/commemorations.ts
+++ b/src/calendar/commemorations.ts
@@ -1,5 +1,75 @@
-// Precedence/occurrence rules for when a lower-ranked feast is commemorated
-// rather than fully displaced by the day's winning feast. Deliberately not
-// designed yet — see plan point 5 ("calendar rules are expected to iterate").
-// Unused until milestone 4.
-export {};
+// Precedence/occurrence resolution: given a day's temporal-cycle standing
+// (calendar/types.ts's TemporalCategory) and a candidate sanctoral feast (if
+// any), which one is actually kept, and whether the loser is commemorated.
+//
+// Modeled as explicit rules per category, not a numeric-weight comparison
+// like the reference engine's occurrence()/concurrence() — see
+// data/calendar/temporal-categories.yml's header for why. The thresholds
+// below are a best-effort reconstruction of the pre-1955 tradition (partly
+// grounded in General Rubrics §15/§16/§23, though that document is itself a
+// later, differently-numbered edition — see that file's header), not
+// verified against a primary source for this exact era. Expect corrections.
+
+import type { FeastClass, TemporalCategory } from './types';
+
+const FEAST_CLASS_ORDER: FeastClass[] = [
+  'simplex',
+  'semiduplex',
+  'duplex',
+  'duplex-majus',
+  'duplex-2-classis',
+  'duplex-1-classis',
+];
+
+export function compareFeastClass(a: FeastClass, b: FeastClass): number {
+  return FEAST_CLASS_ORDER.indexOf(a) - FEAST_CLASS_ORDER.indexOf(b);
+}
+
+export function isAtLeast(rank: FeastClass, threshold: FeastClass): boolean {
+  return compareFeastClass(rank, threshold) >= 0;
+}
+
+export interface OccurrenceResult {
+  winner: 'temporal' | 'sanctoral';
+  commemorated: boolean;
+}
+
+/**
+ * `sanctoral === null` means no feast is assigned to this date at all —
+ * temporal wins trivially, nothing to decide.
+ */
+export function decideOccurrence(temporal: TemporalCategory, sanctoral: FeastClass | null): OccurrenceResult {
+  if (!sanctoral) {
+    return { winner: 'temporal', commemorated: false };
+  }
+
+  switch (temporal) {
+    case 'ordinary-feria':
+      // An ordinary feria has no standing of its own to defend — any real
+      // feast, however low-ranked, is kept in its place.
+      return { winner: 'sanctoral', commemorated: false };
+
+    case 'privileged-feria':
+      // "These ferias are preferred to any feasts whatsoever, and they
+      // admit of no commemoration, except one of the privileged class"
+      // (General Rubrics §23) — read here as: only the very highest class
+      // even gets a mention.
+      return { winner: 'temporal', commemorated: isAtLeast(sanctoral, 'duplex-1-classis') };
+
+    case 'ordinary-sunday':
+      // An ordinary Sunday yields outright only to the highest class; a
+      // Double of the 2nd Class or a Greater Double is kept as a
+      // commemoration instead of displacing the Sunday; anything lower
+      // isn't even mentioned.
+      if (isAtLeast(sanctoral, 'duplex-1-classis')) {
+        return { winner: 'sanctoral', commemorated: false };
+      }
+      return { winner: 'temporal', commemorated: isAtLeast(sanctoral, 'duplex-majus') };
+
+    case 'privileged-sunday':
+      // "A Sunday of the 1st class is preferred to any feast whatsoever"
+      // (General Rubrics §15) — never displaced; commemorated only if the
+      // feast is otherwise of the very top ranks.
+      return { winner: 'temporal', commemorated: isAtLeast(sanctoral, 'duplex-2-classis') };
+  }
+}
diff --git a/src/calendar/feasts.ts b/src/calendar/feasts.ts
index a34ac37..6a761f4 100644
--- a/src/calendar/feasts.ts
+++ b/src/calendar/feasts.ts
@@ -1,6 +1,51 @@
 // Sanctoral occurrence resolution: given a date, which saint(s) are
 // assigned via data/calendar/sanctoral-calendar.yml's day -> saint-id
-// mapping, and their rank/propers/common from data/calendar/saints/<id>.yml.
-// See calendar/temporal.ts for the separate Easter/fixed-date resolution.
-// Unused until milestone 4.
-export {};
+// mapping, and their rank from data/calendar/saints/<id>.yml. See
+// calendar/temporal.ts for the separate temporal-cycle resolution, and
+// calendar/commemorations.ts for how a candidate returned here actually
+// gets weighed against the day's temporal standing.
+//
+// Doesn't resolve clashes *among* multiple saints sharing a date — real
+// practice has its own precedence/commemoration rules for that too, not
+// modeled here. If a day has more than one candidate, calendar/index.ts
+// just picks the highest-ranked as the day's sole sanctoral contender.
+import type { FeastClass } from './types';
+import sanctoralCalendarData from '../data/calendar/sanctoral-calendar.yml';
+
+interface SaintRecord {
+  id: string;
+  name: string;
+  rank: FeastClass;
+  common: string;
+  propers: string | null;
+}
+
+const sanctoralCalendar = sanctoralCalendarData as { days: Record<string, string[]> };
+
+const saintModules = import.meta.glob<{ default: SaintRecord }>('../data/calendar/saints/*.yml', {
+  eager: true,
+});
+
+const saintsById = new Map<string, SaintRecord>();
+for (const mod of Object.values(saintModules)) {
+  saintsById.set(mod.default.id, mod.default);
+}
+
+export interface SanctoralCandidate {
+  id: string;
+  name: string;
+  rank: FeastClass;
+}
+
+/** Which saint(s) (if any) are assigned to this date. `isoDate`'s year is ignored — the sanctoral cycle repeats every civil year. */
+export function getSanctoralCandidatesFor(isoDate: string): SanctoralCandidate[] {
+  const monthDay = isoDate.slice(5);
+  const ids = sanctoralCalendar.days[monthDay] ?? [];
+  return ids.map((id) => {
+    const saint = saintsById.get(id);
+    if (!saint) {
+      throw new Error(`sanctoral-calendar.yml references unknown saint id '${id}'`);
+    }
+    return { id: saint.id, name: saint.name, rank: saint.rank };
+  });
+}
diff --git a/src/calendar/index.ts b/src/calendar/index.ts
index 7c82352..a8281ad 100644
--- a/src/calendar/index.ts
+++ b/src/calendar/index.ts
@@ -1,22 +1,45 @@
-import type { LiturgicalDay } from './types';
+import type { LiturgicalDay, OccurringFeast } from './types';
 import { weekdayOf } from './weekday';
-import { resolveSeason } from './temporal';
+import { resolveSeason, resolveTemporalCategory } from './temporal';
+import { getSanctoralCandidatesFor } from './feasts';
+import { decideOccurrence, compareFeastClass } from './commemorations';
 
 /**
  * Resolves everything about a given day *except* hour content — weekday,
- * season, and any occurring feasts. Season is real (see
- * calendar/temporal.ts); occurring feasts are still a stub until
- * calendar/feasts.ts lands (the sanctoral calendar — which saint, if any,
- * is kept on a given day, and Double-vs-not ranking — is a separate,
- * larger project from temporal-cycle season resolution).
+ * season, temporal precedence category, and any occurring feast. All real
+ * now: `occurring` combines calendar/feasts.ts's sanctoral candidates (if
+ * more than one shares a date, the highest-ranked wins that contest too —
+ * clashes *among* saints aren't otherwise modeled) with
+ * calendar/commemorations.ts's occurrence decision. A feast that's fully
+ * superseded (not even commemorated) doesn't appear in `occurring` at all.
  */
 export function resolveDay(isoDate: string): LiturgicalDay {
-  return {
-    date: isoDate,
-    weekday: weekdayOf(isoDate),
-    season: resolveSeason(isoDate),
-    occurring: [],
-  };
+  const weekday = weekdayOf(isoDate);
+  const season = resolveSeason(isoDate);
+  const temporalCategory = resolveTemporalCategory(isoDate, season, weekday);
+
+  const candidates = getSanctoralCandidatesFor(isoDate);
+  let topCandidate = candidates[0] ?? null;
+  for (const candidate of candidates) {
+    if (compareFeastClass(candidate.rank, topCandidate!.rank) > 0) {
+      topCandidate = candidate;
+    }
+  }
+
+  const occurring: OccurringFeast[] = [];
+  if (topCandidate) {
+    const { winner, commemorated } = decideOccurrence(temporalCategory, topCandidate.rank);
+    if (winner === 'sanctoral' || commemorated) {
+      occurring.push({
+        id: topCandidate.id,
+        name: topCandidate.name,
+        rank: topCandidate.rank,
+        commemorated: winner === 'temporal',
+      });
+    }
+  }
+
+  return { date: isoDate, weekday, season, temporalCategory, occurring };
 }
 
 /**
@@ -31,4 +54,4 @@ export function isSundayOrFeast(day: LiturgicalDay): boolean {
   return day.weekday === 'sunday' || day.occurring.length > 0;
 }
 
-export type { LiturgicalDay, OccurringFeast, Season, Weekday, FeastRank } from './types';
+export type { LiturgicalDay, OccurringFeast, Season, Weekday, FeastClass, TemporalCategory } from './types';
diff --git a/src/calendar/temporal.ts b/src/calendar/temporal.ts
index 0bc23d4..90294c6 100644
--- a/src/calendar/temporal.ts
+++ b/src/calendar/temporal.ts
@@ -11,17 +11,24 @@
 // a season is the temporal-cycle backdrop a day sits on; which saint (if
 // any) is being kept that day, and whether it outranks the season, is a
 // separate, larger project.
-import type { Season } from './types';
+import type { Season, TemporalCategory, Weekday } from './types';
 import { easterSunday } from './easter';
 import { addDays, daysBetween, toIsoDate } from './date-math';
 import fixedDateData from '../data/calendar/fixed-date-calendar.yml';
 import easterOffsetsData from '../data/calendar/easter-offsets.yml';
+import temporalCategoriesData from '../data/calendar/temporal-categories.yml';
 
 const fixedDates = fixedDateData as { dates: Record<string, string> };
 const easterOffsets = easterOffsetsData as {
   ranges: { season: string; fromOffset: number }[];
   days?: Record<string, string>;
 };
+const temporalCategories = temporalCategoriesData as {
+  bySeason: Record<string, { sunday: TemporalCategory; feria: TemporalCategory }>;
+  offsets?: Record<string, TemporalCategory>;
+  offsetRanges?: { fromOffset: number; toOffset: number; category: TemporalCategory }[];
+  fixedDates?: Record<string, TemporalCategory>;
+};
 
 function fixedDateFor(id: string): { month: number; day: number } {
   const entry = Object.entries(fixedDates.dates).find(([, value]) => value === id);
@@ -36,7 +43,7 @@ const CHRISTMAS = fixedDateFor('christmas-day');
 const EPIPHANY = fixedDateFor('epiphany');
 
 /** The Sunday nearest Nov 30 (St. Andrew's Day) — Advent's real start rule. */
-function adventStart(year: number): string {
+export function adventStart(year: number): string {
   const nov30 = `${year}-11-30`;
   const dow = new Date(`${nov30}T00:00:00Z`).getUTCDay(); // 0 = Sunday
   const delta = dow <= 3 ? -dow : 7 - dow;
@@ -94,3 +101,50 @@ export function resolveSeason(isoDate: string): Season {
 
   return seasonFromEasterOffset(daysBetween(easterIso, isoDate));
 }
+
+/** Days from Easter Sunday (negative = before, 0 = Easter, positive = after) for the given date's own year. */
+export function easterOffsetOf(isoDate: string): number {
+  const year = Number(isoDate.slice(0, 4));
+  return daysBetween(toIsoDate(easterSunday(year)), isoDate);
+}
+
+/**
+ * A day's precedence category under the temporal cycle alone — see
+ * calendar/types.ts's TemporalCategory doc comment and
+ * data/calendar/temporal-categories.yml for the reconstruction caveat.
+ *
+ * Sundays never fall on any of the offset/fixed-date overrides below (Ash
+ * Wednesday, Holy Week, the Easter/Pentecost octaves' weekdays, and the
+ * Christmas vigil are all, by construction, not Sundays) except possibly
+ * the Christmas vigil (Dec 24 can land on a Sunday) — and a Sunday's own
+ * privileged/ordinary status should win in that case regardless, so
+ * Sundays are resolved straight from `bySeason` without consulting the
+ * overrides at all.
+ */
+export function resolveTemporalCategory(isoDate: string, season: Season, weekday: Weekday): TemporalCategory {
+  const bySeasonEntry = temporalCategories.bySeason[season];
+  const base = bySeasonEntry ? (weekday === 'sunday' ? bySeasonEntry.sunday : bySeasonEntry.feria) : 'ordinary-feria';
+  if (weekday === 'sunday') {
+    return base;
+  }
+
+  const fixedOverride = temporalCategories.fixedDates?.[isoDate.slice(5)];
+  if (fixedOverride) {
+    return fixedOverride;
+  }
+
+  const offset = easterOffsetOf(isoDate);
+
+  const singleOverride = temporalCategories.offsets?.[String(offset)];
+  if (singleOverride) {
+    return singleOverride;
+  }
+
+  for (const range of temporalCategories.offsetRanges ?? []) {
+    if (offset >= range.fromOffset && offset <= range.toOffset) {
+      return range.category;
+    }
+  }
+
+  return base;
+}
diff --git a/src/calendar/types.ts b/src/calendar/types.ts
index 895a5f2..54157be 100644
--- a/src/calendar/types.ts
+++ b/src/calendar/types.ts
@@ -36,18 +36,35 @@ export type Weekday =
 // third case shows up and the duplication starts to hurt.
 export type Season = string;
 
-// Open-ended on purpose — the actual ranking scheme (double/semidouble/simple,
-// or whatever the finalized rank system turns out to be) is a rule decision for
-// milestone 4, not a type decision for milestone 0.
-export type FeastRank = string;
+// The old (pre-1955) rank scale, six levels, low to high. Deliberately a
+// closed union rather than an open string like Season — the whole point of
+// calendar/commemorations.ts's decideOccurrence is to compare two of these
+// with explicit, readable rules, which only works if the set of values is
+// fixed and known. See calendar/types.ts's TemporalCategory doc comment for
+// the other half of that comparison.
+export type FeastClass = 'simplex' | 'semiduplex' | 'duplex' | 'duplex-majus' | 'duplex-2-classis' | 'duplex-1-classis';
+
+// A day's own precedence class *before* any sanctoral feast is considered —
+// i.e. what the temporal cycle alone says this day is entitled to. This is
+// coarser than `season` on purpose: several different seasons share the
+// same precedence behavior (Advent/Septuagesima/Lent/Passiontide Sundays
+// are all "privileged" in the same way; Epiphanytide/Trinitytide Sundays
+// are all "ordinary" in the same way), and decideOccurrence only cares
+// about that behavior, not which season produced it. See
+// data/calendar/temporal-categories.yml for which season maps to which
+// category — reconstructed from general knowledge of the pre-1955
+// tradition, not yet verified against a primary source, so expect
+// corrections.
+export type TemporalCategory = 'ordinary-feria' | 'privileged-feria' | 'ordinary-sunday' | 'privileged-sunday';
 
 export interface OccurringFeast {
   id: string;
   name: string;
-  rank: FeastRank;
+  rank: FeastClass;
   commemorated: boolean;
-  // Placeholder for the first/second-Vespers overlap wrinkle — unresolved until
-  // milestone 4 actually needs it.
+  // Set by calendar/vespers.ts's resolveEveningDay when this feast's First
+  // Vespers is being anticipated this evening (i.e. this OccurringFeast
+  // belongs to *tomorrow*, but is winning tonight's Vespers/Compline).
   vespersFrom?: 'today' | 'firstVespersOfTomorrow';
 }
 
@@ -57,6 +74,8 @@ export interface LiturgicalDay {
   weekday: Weekday;
   /** Real temporal-cycle season, computed via calendar/temporal.ts. */
   season: Season;
-  /** Always [] until milestone 4 wires up calendar/feasts.ts. */
+  /** This day's own precedence class, before any sanctoral feast wins or loses against it. */
+  temporalCategory: TemporalCategory;
+  /** The feast(s) actually occurring — real, via calendar/feasts.ts + calendar/commemorations.ts. */
   occurring: OccurringFeast[];
 }
diff --git a/src/data/calendar/saints/all-saints.yml b/src/data/calendar/saints/all-saints.yml
new file mode 100644
index 0000000..b6b2bd7
--- /dev/null
+++ b/src/data/calendar/saints/all-saints.yml
@@ -0,0 +1,7 @@
+# Verified against Divinum Officium (web/www/horas/Latin/Sancti/11-01.txt,
+# untagged/default [Rank] block).
+id: all-saints
+name: "All Saints"
+rank: duplex-1-classis
+common: proper-to-all-saints
+propers: null
diff --git a/src/data/calendar/saints/assumption.yml b/src/data/calendar/saints/assumption.yml
new file mode 100644
index 0000000..3d1ed2d
--- /dev/null
+++ b/src/data/calendar/saints/assumption.yml
@@ -0,0 +1,7 @@
+# Verified against Divinum Officium (web/www/horas/Latin/Sancti/08-15.txt,
+# untagged/default [Rank] block).
+id: assumption
+name: "The Assumption of the Blessed Virgin Mary"
+rank: duplex-1-classis
+common: common-of-the-bvm
+propers: null
diff --git a/src/data/calendar/saints/example-confessor.yml b/src/data/calendar/saints/example-confessor.yml
index e0c1e6e..67c3599 100644
--- a/src/data/calendar/saints/example-confessor.yml
+++ b/src/data/calendar/saints/example-confessor.yml
@@ -1,9 +1,8 @@
-# PLACEHOLDER — demonstrates the saint-record shape only. Not a real saint,
-# not a real rank, not a real day assignment (see sanctoral-calendar.yml for
-# where this id gets pointed at a day). Which day(s) point here can change
-# freely without ever touching this file.
+# PLACEHOLDER — demonstrates the saint-record shape only, deliberately not
+# mapped to any day in sanctoral-calendar.yml (which now holds real,
+# verified entries — see that file). Not a real saint, not a real rank.
 id: example-confessor
 name: "Example Confessor (placeholder)"
-rank: placeholder # real rank scheme is a milestone 4 decision, not made yet
+rank: simplex # placeholder value
 common: common-of-a-confessor-not-bishop
-propers: null # set to a propers id once/if this saint has a full proper
+propers: null
diff --git a/src/data/calendar/saints/immaculate-conception.yml b/src/data/calendar/saints/immaculate-conception.yml
new file mode 100644
index 0000000..41a8a9c
--- /dev/null
+++ b/src/data/calendar/saints/immaculate-conception.yml
@@ -0,0 +1,7 @@
+# Verified against Divinum Officium (web/www/horas/Latin/Sancti/12-08.txt,
+# untagged/default [Rank] block).
+id: immaculate-conception
+name: "The Immaculate Conception of the Blessed Virgin Mary"
+rank: duplex-1-classis
+common: common-of-the-bvm
+propers: null
diff --git a/src/data/calendar/saints/nativity-bvm.yml b/src/data/calendar/saints/nativity-bvm.yml
new file mode 100644
index 0000000..9666bbc
--- /dev/null
+++ b/src/data/calendar/saints/nativity-bvm.yml
@@ -0,0 +1,7 @@
+# Verified against Divinum Officium (web/www/horas/Latin/Sancti/09-08.txt,
+# untagged/default [Rank] block).
+id: nativity-bvm
+name: "The Nativity of the Blessed Virgin Mary"
+rank: duplex-2-classis
+common: common-of-the-bvm
+propers: null
diff --git a/src/data/calendar/saints/st-augustine.yml b/src/data/calendar/saints/st-augustine.yml
new file mode 100644
index 0000000..d0752f9
--- /dev/null
+++ b/src/data/calendar/saints/st-augustine.yml
@@ -0,0 +1,9 @@
+# Verified against Divinum Officium (web/www/horas/Latin/Sancti/08-28.txt,
+# untagged/default [Rank] block — that file's own "(sed rubrica 1930)"
+# alternate raises this to Duplex majus, but 1930 postdates the pre-1910
+# calendar this app targets, so the plain default is used instead.
+id: st-augustine
+name: "St. Augustine of Hippo, Bishop, Confessor and Doctor of the Church"
+rank: duplex
+common: common-of-a-confessor-bishop
+propers: null
diff --git a/src/data/calendar/saints/st-lawrence.yml b/src/data/calendar/saints/st-lawrence.yml
new file mode 100644
index 0000000..a2e96c5
--- /dev/null
+++ b/src/data/calendar/saints/st-lawrence.yml
@@ -0,0 +1,8 @@
+# Verified against Divinum Officium (web/www/horas/Latin/Sancti/08-10.txt,
+# untagged/default [Rank] block — the pre-1955 value, not the "(sed rubrica
+# 196)" alternate given alongside it in that file).
+id: st-lawrence
+name: "St. Lawrence, Martyr"
+rank: duplex-2-classis
+common: common-of-a-martyr
+propers: null
diff --git a/src/data/calendar/saints/st-michael.yml b/src/data/calendar/saints/st-michael.yml
new file mode 100644
index 0000000..ce7fca6
--- /dev/null
+++ b/src/data/calendar/saints/st-michael.yml
@@ -0,0 +1,9 @@
+# Verified against Divinum Officium (web/www/horas/Latin/Sancti/09-29.txt,
+# untagged/default [Rank] block — that file's own "(sed rubrica tridentina)"
+# alternate gives Duplex II classis instead; the plain default is used here
+# for consistency with how every other saint file in this set was read).
+id: st-michael
+name: "St. Michael the Archangel"
+rank: duplex-1-classis
+common: common-of-an-angel
+propers: null
diff --git a/src/data/calendar/sanctoral-calendar.yml b/src/data/calendar/sanctoral-calendar.yml
index 8161840..0299182 100644
--- a/src/data/calendar/sanctoral-calendar.yml
+++ b/src/data/calendar/sanctoral-calendar.yml
@@ -5,8 +5,17 @@
 # (data/calendar/saints/<id>.yml) never changes — it's the same record
 # whichever day points at it.
 #
-# PLACEHOLDER: "01-01" -> example-confessor is fake data proving the shape,
-# not a real calendar assignment. Real sanctoral content-authoring
-# (a pre-1910-leaning calendar, per the plan) is a separate task.
+# A small, growable subset, not a full year — see the plan discussion this
+# came out of. Each entry's rank is verified directly against Divinum
+# Officium (see the saint's own file for which one) rather than
+# reconstructed from memory, unlike data/calendar/temporal-categories.yml's
+# privileged-day lists. `example-confessor` is deliberately NOT mapped to
+# any day here — it's fictional, kept only to document the record shape.
 days:
-  "01-01": [example-confessor]
+  "08-10": [st-lawrence]
+  "08-15": [assumption]
+  "08-28": [st-augustine]
+  "09-08": [nativity-bvm]
+  "09-29": [st-michael]
+  "11-01": [all-saints]
+  "12-08": [immaculate-conception]
diff --git a/src/data/calendar/temporal-categories.yml b/src/data/calendar/temporal-categories.yml
new file mode 100644
index 0000000..6a554c3
--- /dev/null
+++ b/src/data/calendar/temporal-categories.yml
@@ -0,0 +1,59 @@
+# A day's precedence category under the temporal cycle alone, before any
+# sanctoral feast is weighed against it — see calendar/types.ts's
+# TemporalCategory doc comment for why this is coarser than `season`, and
+# calendar/commemorations.ts for how it's actually used (decideOccurrence).
+#
+# RECONSTRUCTED, not verified against a primary source: the two rubric
+# documents actually present in the reference engine
+# (divinum-officium-reference/web/www/horas/Help/Rubrics/) are both later
+# editions (~1960) with a different, renumbered classification than the
+# pre-1955 system this app targets. This is a best-effort reconstruction
+# from general knowledge of that older tradition — expect corrections.
+#
+# `bySeason` gives the default for an ordinary Sunday/feria falling in that
+# season. `offsets`/`offsetRanges` (from Easter, same convention as
+# easter-offsets.yml) and `fixedDates` (MM-DD) override that default for
+# specific privileged ferias that don't line up with a whole season.
+#
+# Known gap: Advent Ember days (Wed/Fri/Sat after Dec 13) and September
+# Ember days (Wed/Fri/Sat near the Exaltation of the Cross, Sept 14) are
+# NOT modeled here — both are fixed-calendar "nearest such-and-such a date"
+# rules, not a plain offset, and weren't worth guessing at without a source.
+# Lent's and Pentecost's Ember days *are* modeled, since those are clean
+# Easter offsets.
+bySeason:
+  advent: { sunday: privileged-sunday, feria: ordinary-feria }
+  # Christmastide's ferias are folded into the privileged-feria default for
+  # the whole season as a simplification of "the Octave of Christmas (Dec
+  # 25 - Jan 1) is privileged" — slightly overbroad into Jan 2-5, harmless.
+  christmastide: { sunday: privileged-sunday, feria: privileged-feria }
+  epiphanytide: { sunday: ordinary-sunday, feria: ordinary-feria }
+  septuagesima: { sunday: privileged-sunday, feria: ordinary-feria }
+  lent: { sunday: privileged-sunday, feria: ordinary-feria }
+  passiontide: { sunday: privileged-sunday, feria: ordinary-feria }
+  eastertide: { sunday: privileged-sunday, feria: ordinary-feria }
+  ascensiontide: { sunday: ordinary-sunday, feria: ordinary-feria }
+  pentecost: { sunday: privileged-sunday, feria: ordinary-feria }
+  trinitytide: { sunday: ordinary-sunday, feria: ordinary-feria }
+  # Corpus Christi/Sacred Heart are themselves "feasts of the Lord" that
+  # shouldn't cede to an ordinary saint — modeled here as privileged-feria
+  # (rather than injecting a separate synthetic FeastClass) since with the
+  # small sanctoral calendar this app has, a real clash is unlikely and the
+  # practical effect (temporal wins) is the same either way.
+  corpus-christi: { sunday: ordinary-sunday, feria: privileged-feria }
+  sacred-heart: { sunday: ordinary-sunday, feria: privileged-feria }
+
+offsets:
+  -46: privileged-feria # Ash Wednesday
+  -39: privileged-feria # Lent Ember Wednesday (Wed after 1st Sunday of Lent)
+  -37: privileged-feria # Lent Ember Friday
+  -36: privileged-feria # Lent Ember Saturday
+  48: privileged-feria # Vigil of Pentecost
+
+offsetRanges:
+  - { fromOffset: -6, toOffset: -1, category: privileged-feria } # ferias of Holy Week
+  - { fromOffset: 1, toOffset: 6, category: privileged-feria } # Easter octave
+  - { fromOffset: 50, toOffset: 55, category: privileged-feria } # Pentecost octave (also covers Pentecost's own Ember days)
+
+fixedDates:
+  "12-24": privileged-feria # Vigil of Christmas
diff --git a/src/hours/antiphon.ts b/src/hours/antiphon.ts
index 8c24a52..7777045 100644
--- a/src/hours/antiphon.ts
+++ b/src/hours/antiphon.ts
@@ -1,4 +1,5 @@
 import type { OccurringFeast } from '../calendar/types';
+import { isAtLeast } from '../calendar/commemorations';
 
 export interface SplitAntiphon {
   incipit: string;
@@ -30,13 +31,13 @@ export function splitAntiphon(text: string): SplitAntiphon {
  * said before the psalms (the full text is always said after, regardless
  * of rank).
  *
- * FeastRank is an open string with no defined hierarchy yet (see
- * calendar/types.ts), and `occurring` is always [] until milestone 4 wires
- * up real feast data — so this can only ever return false today. That's
- * the *correct* answer for every day currently reachable (a plain ferial
- * day is below Double), not a stub papering over missing logic; it starts
- * doing real work the moment FeastRank has an ordering to compare against.
+ * Only asks whether the day's *winning* feast (not a merely-commemorated
+ * one) is Double-or-higher — a privileged Sunday with nothing occurring,
+ * or a commemorated low-rank saint, both correctly return false here. A
+ * Sunday being "privileged" doesn't by itself make this true; that's a
+ * `TemporalCategory` question, not a `FeastClass` one.
  */
-export function isDoubleOrHigher(_occurring: OccurringFeast[]): boolean {
-  return false;
+export function isDoubleOrHigher(occurring: OccurringFeast[]): boolean {
+  const winner = occurring.find((feast) => !feast.commemorated);
+  return winner ? isAtLeast(winner.rank, 'duplex') : false;
 }
diff --git a/tests/calendar/commemorations.test.ts b/tests/calendar/commemorations.test.ts
new file mode 100644
index 0000000..33184aa
--- /dev/null
+++ b/tests/calendar/commemorations.test.ts
@@ -0,0 +1,69 @@
+import { describe, expect, it } from 'vitest';
+import { decideOccurrence, compareFeastClass, isAtLeast } from '../../src/calendar/commemorations';
+
+describe('compareFeastClass / isAtLeast', () => {
+  it('orders the six ranks low to high', () => {
+    expect(compareFeastClass('simplex', 'duplex-1-classis')).toBeLessThan(0);
+    expect(compareFeastClass('duplex-1-classis', 'simplex')).toBeGreaterThan(0);
+    expect(compareFeastClass('duplex', 'duplex')).toBe(0);
+  });
+
+  it('isAtLeast is inclusive of the threshold itself', () => {
+    expect(isAtLeast('duplex-majus', 'duplex-majus')).toBe(true);
+    expect(isAtLeast('duplex', 'duplex-majus')).toBe(false);
+  });
+});
+
+describe('decideOccurrence', () => {
+  it('temporal wins trivially when nothing is assigned to the date', () => {
+    expect(decideOccurrence('ordinary-feria', null)).toEqual({ winner: 'temporal', commemorated: false });
+    expect(decideOccurrence('privileged-sunday', null)).toEqual({ winner: 'temporal', commemorated: false });
+  });
+
+  it('an ordinary feria always yields to any real feast, uncommemorated', () => {
+    expect(decideOccurrence('ordinary-feria', 'simplex')).toEqual({ winner: 'sanctoral', commemorated: false });
+    expect(decideOccurrence('ordinary-feria', 'duplex-1-classis')).toEqual({ winner: 'sanctoral', commemorated: false });
+  });
+
+  it('a privileged feria yields to nothing short of the top class', () => {
+    expect(decideOccurrence('privileged-feria', 'duplex-2-classis')).toEqual({
+      winner: 'temporal',
+      commemorated: false,
+    });
+    expect(decideOccurrence('privileged-feria', 'duplex-1-classis')).toEqual({
+      winner: 'temporal',
+      commemorated: true,
+    });
+  });
+
+  it('an ordinary Sunday yields outright only to the top class, and is commemorated by the two ranks below it', () => {
+    expect(decideOccurrence('ordinary-sunday', 'duplex-1-classis')).toEqual({
+      winner: 'sanctoral',
+      commemorated: false,
+    });
+    expect(decideOccurrence('ordinary-sunday', 'duplex-2-classis')).toEqual({
+      winner: 'temporal',
+      commemorated: true,
+    });
+    expect(decideOccurrence('ordinary-sunday', 'duplex-majus')).toEqual({
+      winner: 'temporal',
+      commemorated: true,
+    });
+    expect(decideOccurrence('ordinary-sunday', 'duplex')).toEqual({ winner: 'temporal', commemorated: false });
+  });
+
+  it('a privileged Sunday is never displaced, and only the top two ranks are even commemorated', () => {
+    expect(decideOccurrence('privileged-sunday', 'duplex-1-classis')).toEqual({
+      winner: 'temporal',
+      commemorated: true,
+    });
+    expect(decideOccurrence('privileged-sunday', 'duplex-2-classis')).toEqual({
+      winner: 'temporal',
+      commemorated: true,
+    });
+    expect(decideOccurrence('privileged-sunday', 'duplex-majus')).toEqual({
+      winner: 'temporal',
+      commemorated: false,
+    });
+  });
+});
diff --git a/tests/hours/compline.test.ts b/tests/hours/compline.test.ts
index d9a175a..994ab0a 100644
--- a/tests/hours/compline.test.ts
+++ b/tests/hours/compline.test.ts
@@ -1,8 +1,8 @@
 import { describe, expect, it } from 'vitest';
 import { resolveOrdo } from '../../src/hours';
 
-const MONDAY = '2026-08-10';
-const SUNDAY = '2026-08-09';
+const MONDAY = '2026-06-15';
+const SUNDAY = '2026-06-14';
 
 describe('resolveOrdo("compline", ...)', () => {
   it('is implemented, with fixed psalms 4, 90, 133 regardless of weekday', () => {
diff --git a/tests/hours/prime.test.ts b/tests/hours/prime.test.ts
index d16537a..6122204 100644
--- a/tests/hours/prime.test.ts
+++ b/tests/hours/prime.test.ts
@@ -5,7 +5,7 @@ import { resolveOrdo } from '../../src/hours';
 // whole psalms (1, 2, 6), the first of which (Psalm 1) has real sample
 // content in data/psalms/001.yml, so these tests can check real verse data
 // without needing every psalm authored.
-const MONDAY = '2026-08-10';
+const MONDAY = '2026-06-15';
 const SUNDAY = '2026-08-09';
 
 describe('resolveOrdo("prime", ...)', () => {