diff --git a/TODO.md b/TODO.md
index dfbd8db..7194d8a 100644
--- a/TODO.md
+++ b/TODO.md
@@ -116,6 +116,37 @@ instead of the correct plain temporal label. Fixed by having
 `resolveActiveOctave` call as `resolveOfficeWinner`, so the two can never
 disagree about which identity governs a day.
 
+### Dec 26-30's own Sunday rule — done (2026-08), a deliberate remix
+
+Per direct instruction — a real, admitted departure from Monastic 1617
+itself, not something independently verified live the way almost
+everything else here is: within Dec 26-30 inclusive (every year contains
+exactly one real Sunday somewhere in that 5-day window), whichever date
+is the actual Sunday always celebrates Nat1-0 (the Sunday within the
+Octave of the Nativity) outright, with **no rank check at all** — even a
+Duplex-II-classis saint (Stephen/John/Innocents) is fully displaced, not
+just commemorated. The displaced saint isn't commemorated in place
+either; he reappears in full as the day's own winner on Dec 30 instead —
+the block's one date with no fixed saint of its own. On a year Dec 30
+itself is the actual Sunday, nothing is displaced and it resolves clean.
+
+This intentionally diverges from what Monastic 1617 itself does on Dec
+26-29 (there, live-verified, the Duplex-II-classis saint wins and the
+Sunday is merely commemorated — see the December-sanctoral tests) and
+from the general `privileged-sunday` transfer rule elsewhere in this app
+(which only transfers ranks below duplex-majus and commemorates
+everything at or above it in place) — this window transfers *every*
+rank, full stop, and never commemorates the loser in place.
+
+Implemented as its own dedicated, narrowly-scoped layer
+(`calendar/index.ts`'s `applyChristmasOctaveSunday`, same architectural
+pattern as `applyMarianSaturday`/`applyChristTheKing`) rather than by
+changing the shared `decideOccurrence`/transfer machinery other Sundays
+rely on — deliberately contained blast radius. Dec 31 and everything
+outside Dec 26-30 is untouched; christmastide's Sunday there still uses
+the plain `ordinary-sunday` category as before (St. Silvester still
+beats the Sunday on Dec 31, live-verified, unaffected by this).
+
 ### Benedictus antiphons for temporal (non-sanctoral) days — done
 
 `<id>-benedictus-antiphon.yml` in `data/propers/temporal/` — all 54
diff --git a/src/calendar/index.ts b/src/calendar/index.ts
index eb9d49e..5e07969 100644
--- a/src/calendar/index.ts
+++ b/src/calendar/index.ts
@@ -103,6 +103,7 @@ export function resolveDay(isoDate: string): LiturgicalDay {
   winner = applyOctaves(isoDate, winner, commemorations);
   winner = applyMarianSaturday(isoDate, weekday, temporalCategory, winner, commemorations);
   winner = applyChristTheKing(isoDate, winner, commemorations);
+  winner = applyChristmasOctaveSunday(isoDate, weekday, winner, commemorations);
 
   return { date: isoDate, weekday, season, temporalCategory, winner, commemorations };
 }
@@ -137,6 +138,65 @@ function applyChristTheKing(isoDate: string, winner: DayWinner, commemorations:
   return { kind: 'temporal', id: 'christ-the-king' };
 }
 
+/**
+ * Per direct instruction, scoped to exactly Dec 26-30 — every year
+ * contains exactly one real Sunday somewhere in that 5-day window.
+ *
+ * On whichever of Dec 26-29 it actually falls, Nat1-0 (the Sunday within
+ * the Octave of the Nativity) always wins outright, full stop — no rank
+ * check at all, a deliberate departure from what Monastic 1617 itself
+ * does there (a Duplex-II-classis saint — Stephen/John/Innocents — beats
+ * the Sunday there in the real engine; see TODO.md). The displaced
+ * saint isn't even commemorated in place; he reappears in full on Dec
+ * 30 instead — the block's one date with no fixed saint of its own —
+ * rather than the general privileged-sunday transfer/commemorate split
+ * used everywhere else. Dec 30 itself, on a year it's the *actual*
+ * Sunday (nothing displaced), resolves normally: Nat1-0 wins clean since
+ * nothing was ever assigned there to begin with.
+ */
+function applyChristmasOctaveSunday(
+  isoDate: string,
+  weekday: ReturnType<typeof weekdayOf>,
+  winner: DayWinner,
+  commemorations: Commemoration[],
+): DayWinner {
+  const [year, monthStr, dayStr] = isoDate.split('-');
+  if (monthStr !== '12') {
+    return winner;
+  }
+  const day = Number(dayStr);
+
+  if (day >= 26 && day <= 29 && weekday === 'sunday') {
+    return { kind: 'temporal', id: 'christmas-octave-sunday' };
+  }
+
+  if (day === 30) {
+    for (let d = 26; d <= 29; d++) {
+      const candidateDate = `${year}-12-${String(d).padStart(2, '0')}`;
+      if (weekdayOf(candidateDate) !== 'sunday') {
+        continue;
+      }
+      const displaced = getSanctoralCandidatesFor(candidateDate)[0];
+      if (!displaced) {
+        break;
+      }
+      // Drop the now-redundant "Octave of <displaced>" commemoration —
+      // applyOctaves already pushed one above, using Dec 30's *original*
+      // temporal winner; now that the displaced saint is winning the day
+      // outright instead, commemorating his own octave alongside himself
+      // would be the same redundancy applyOctaves' own `isSelf` check
+      // already avoids on a saint's actual day.
+      const idx = commemorations.findIndex((c) => c.kind === 'octave' && c.id === displaced.id);
+      if (idx !== -1) {
+        commemorations.splice(idx, 1);
+      }
+      return { kind: 'sanctoral', id: displaced.id, name: displaced.name, rank: displaced.rank };
+    }
+  }
+
+  return winner;
+}
+
 /**
  * Layered on top of everything above, same spirit as applyOctaves: never
  * changes decideOccurrence's own rules, just relabels the result on a free
diff --git a/tests/calendar/christmas-octave-sunday.test.ts b/tests/calendar/christmas-octave-sunday.test.ts
new file mode 100644
index 0000000..eac5c61
--- /dev/null
+++ b/tests/calendar/christmas-octave-sunday.test.ts
@@ -0,0 +1,119 @@
+import { describe, expect, it } from 'vitest';
+import { resolveDay } from '../../src/calendar';
+
+// Per direct instruction: within Dec 26-30 inclusive, exactly one date is
+// genuinely the Sunday within the Octave of the Nativity each year. That
+// date always wins outright as Nat1-0 (christmas-octave-sunday), no rank
+// check at all -- a deliberate departure from Monastic 1617 itself, which
+// lets a Duplex-II-classis saint (Stephen/John/Innocents) beat the Sunday
+// there (see tests/calendar/december-sanctoral.test.ts's own Dec 31/St.
+// Silvester test for where that live-verified rule is still real, outside
+// this window). The displaced saint isn't commemorated in place -- he
+// reappears in full on Dec 30, the block's one date with no fixed saint of
+// its own.
+describe('applyChristmasOctaveSunday', () => {
+  it('Sunday on Dec 26 (St. Stephen, Duplex II. classis): Nat1-0 wins outright, Stephen not even commemorated, and reappears in full on Dec 30', () => {
+    const sunday = resolveDay('2021-12-26');
+    expect(sunday.weekday).toBe('sunday');
+    expect(sunday.winner).toEqual({ kind: 'temporal', id: 'christmas-octave-sunday' });
+    expect(sunday.commemorations.some((c) => c.kind === 'sanctoral')).toBe(false);
+
+    const dec30 = resolveDay('2021-12-30');
+    expect(dec30.winner).toEqual({
+      kind: 'sanctoral',
+      id: 'st-stephen-protomartyr',
+      name: 'St. Stephen, Protomartyr',
+      rank: 'duplex-2-classis',
+    });
+    // The now-redundant "Octave of St. Stephen" commemoration (which
+    // would otherwise have been pushed by applyOctaves, using Dec 30's
+    // *original* temporal winner) is dropped -- he's the day's own
+    // winner now, not merely commemorated alongside himself.
+    expect(dec30.commemorations.some((c) => c.kind === 'octave' && c.id === 'st-stephen-protomartyr')).toBe(false);
+    // The other, still-genuinely-separate octaves remain.
+    expect(dec30.commemorations.map((c) => (c.kind === 'octave' ? c.id : undefined))).toEqual([
+      'christmas-day',
+      'st-john-apostle',
+      'holy-innocents',
+    ]);
+  });
+
+  it('Sunday on Dec 27 (St. John, Duplex II. classis): same shape', () => {
+    const sunday = resolveDay('2020-12-27');
+    expect(sunday.winner).toEqual({ kind: 'temporal', id: 'christmas-octave-sunday' });
+
+    const dec30 = resolveDay('2020-12-30');
+    expect(dec30.winner).toEqual({
+      kind: 'sanctoral',
+      id: 'st-john-apostle',
+      name: 'St. John, Apostle and Evangelist',
+      rank: 'duplex-2-classis',
+    });
+    expect(dec30.commemorations.some((c) => c.kind === 'octave' && c.id === 'st-john-apostle')).toBe(false);
+  });
+
+  it('Sunday on Dec 28 (Holy Innocents, Duplex II. classis): same shape', () => {
+    const sunday = resolveDay('2025-12-28');
+    expect(sunday.winner).toEqual({ kind: 'temporal', id: 'christmas-octave-sunday' });
+
+    const dec30 = resolveDay('2025-12-30');
+    expect(dec30.winner).toEqual({
+      kind: 'sanctoral',
+      id: 'holy-innocents',
+      name: 'The Holy Innocents, Martyrs',
+      rank: 'duplex-2-classis',
+    });
+    expect(dec30.commemorations.some((c) => c.kind === 'octave' && c.id === 'holy-innocents')).toBe(false);
+  });
+
+  it('Sunday on Dec 29 (St. Thomas Becket, only Semiduplex): still fully displaced to Dec 30, same as the Duplex-II-classis saints -- rank never matters for this rule', () => {
+    const sunday = resolveDay('2024-12-29');
+    expect(sunday.winner).toEqual({ kind: 'temporal', id: 'christmas-octave-sunday' });
+
+    const dec30 = resolveDay('2024-12-30');
+    expect(dec30.winner).toEqual({
+      kind: 'sanctoral',
+      id: 'st-thomas-becket',
+      name: 'St. Thomas of Canterbury, Bishop and Martyr',
+      rank: 'semiduplex',
+    });
+  });
+
+  it('Sunday on Dec 30 itself: nothing to displace, resolves clean', () => {
+    const dec30 = resolveDay('2029-12-30');
+    expect(dec30.weekday).toBe('sunday');
+    expect(dec30.winner).toEqual({ kind: 'temporal', id: 'christmas-octave-sunday' });
+    // All four octaves still commemorated normally -- nothing was ever
+    // assigned to Dec 30 itself, so there's nothing to drop.
+    expect(dec30.commemorations.map((c) => (c.kind === 'octave' ? c.id : undefined))).toEqual([
+      'christmas-day',
+      'st-stephen-protomartyr',
+      'st-john-apostle',
+      'holy-innocents',
+    ]);
+  });
+
+  it('Dec 26-29 keep their own fixed saint outright on a year none of them is the Sunday, unaffected by this rule at all', () => {
+    // 2029: the Sunday is Dec 30, so Dec 26-29 each just resolve normally.
+    expect(resolveDay('2029-12-26').winner).toEqual({
+      kind: 'sanctoral',
+      id: 'st-stephen-protomartyr',
+      name: 'St. Stephen, Protomartyr',
+      rank: 'duplex-2-classis',
+    });
+    expect(resolveDay('2029-12-29').winner).toEqual({
+      kind: 'sanctoral',
+      id: 'st-thomas-becket',
+      name: 'St. Thomas of Canterbury, Bishop and Martyr',
+      rank: 'semiduplex',
+    });
+  });
+
+  it('never touches Dec 31 or Jan 1, outside its own Dec 26-30 scope', () => {
+    // St. Silvester (Dec 31) still wins outright via the plain
+    // ordinary-sunday rule even in a year Dec 31 is the Sunday -- see
+    // tests/calendar/december-sanctoral.test.ts for this one directly.
+    const dec31 = resolveDay('2023-12-31');
+    expect(dec31.winner.kind).toBe('sanctoral');
+  });
+});
diff --git a/tests/calendar/december-sanctoral.test.ts b/tests/calendar/december-sanctoral.test.ts
index 756dcd1..db90c98 100644
--- a/tests/calendar/december-sanctoral.test.ts
+++ b/tests/calendar/december-sanctoral.test.ts
@@ -108,34 +108,15 @@ describe('December sanctoral pull (first pass, 12/12)', () => {
     ]);
   });
 
-  it('the Christmas Octave Sunday loses outright to a Duplex+ Comites Christi feast when the two collide -- ordinary-sunday, not privileged-sunday', () => {
-    // Corrects a real bug: christmastide's Sunday was originally coded
-    // privileged-sunday (Advent's own tier), which would have kept the
-    // Sunday primary here with the saint merely commemorated -- backwards
-    // from the live Monastic 1617 engine, which was checked directly
-    // against all three Duplex-II-classis collision dates and shows the
-    // saint winning outright every time, the Sunday only commemorated.
-    const stephenSunday = resolveDay('2021-12-26');
-    expect(stephenSunday.winner).toEqual({
-      kind: 'sanctoral',
-      id: 'st-stephen-protomartyr',
-      name: 'St. Stephen, Protomartyr',
-      rank: 'duplex-2-classis',
-    });
-    expect(stephenSunday.commemorations).toContainEqual({ kind: 'temporal', id: 'christmas-octave-sunday' });
-
-    const innocentsSunday = resolveDay('2025-12-28');
-    expect(innocentsSunday.winner).toEqual({
-      kind: 'sanctoral',
-      id: 'holy-innocents',
-      name: 'The Holy Innocents, Martyrs',
-      rank: 'duplex-2-classis',
-    });
-    expect(innocentsSunday.commemorations).toContainEqual({ kind: 'temporal', id: 'christmas-octave-sunday' });
-
-    // A plain Duplex (not just Duplex-II-classis) is still enough: St.
-    // Silvester (Dec 31) wins outright the same way when it falls on the
-    // Sunday, also live-verified.
+  it('the underlying ordinary-sunday classification still governs Dec 31 (outside the Dec 26-30 window applyChristmasOctaveSunday owns) -- a Duplex still beats the Sunday there, live-verified', () => {
+    // St. Silvester (Dec 31) wins outright when it falls on the Sunday --
+    // unaffected by applyChristmasOctaveSunday, which is deliberately
+    // scoped to exactly Dec 26-30 (see tests/calendar/
+    // christmas-octave-sunday.test.ts for that window's own, different
+    // rule). This is what confirms christmastide's own `ordinary-sunday`
+    // temporal-category classification (see data/calendar/
+    // temporal-categories.yml) is still real and still needed, not
+    // superseded outright by the newer Dec 26-30 rule.
     const silvesterSunday = resolveDay('2023-12-31');
     expect(silvesterSunday.winner).toEqual({
       kind: 'sanctoral',
@@ -146,11 +127,14 @@ describe('December sanctoral pull (first pass, 12/12)', () => {
     expect(silvesterSunday.commemorations).toContainEqual({ kind: 'temporal', id: 'christmas-octave-sunday' });
   });
 
-  it('the Christmas Octave Sunday still keeps a lesser feast from winning, same as before the ordinary-sunday fix', () => {
+  it('the Christmas Octave Sunday still keeps a lesser feast from winning, now via applyChristmasOctaveSunday specifically', () => {
     // St. Thomas Becket (Semiduplex, Dec 29) transfers off the Sunday
-    // rather than winning it -- unaffected by the privileged->ordinary
-    // change, since both tiers transfer a Semiduplex here; this test
-    // guards against a regression the other direction.
+    // rather than winning it -- per calendar/index.ts's
+    // applyChristmasOctaveSunday (see its own dedicated test file), which
+    // now unconditionally wins the Sunday over *any* rank within Dec
+    // 26-29, superseding the plain ordinary-sunday rule for this narrow
+    // window specifically (that rule is still real outside it -- see the
+    // Dec 31/St. Silvester test above).
     const beckettSunday = resolveDay('2024-12-29');
     expect(beckettSunday.winner.kind).toBe('temporal');
     expect(beckettSunday.commemorations.some((c) => c.kind === 'sanctoral')).toBe(false);
diff --git a/tests/hours/lauds.test.ts b/tests/hours/lauds.test.ts
index 6baddc2..03d86bc 100644
--- a/tests/hours/lauds.test.ts
+++ b/tests/hours/lauds.test.ts
@@ -141,7 +141,11 @@ describe('resolveOrdo("lauds", ...)', () => {
   });
 
   it('resolves a real Benedictus antiphon for christmas-octave-sunday too, pulled from the reference engine\'s own dedicated Tempora/Nat1-0 proper rather than any individual octave-day saint\'s page', () => {
-    const ordo = resolveOrdo('lauds', '2026-12-30');
+    // 2029-12-30 specifically: the year Dec 30 itself is the real Sunday
+    // within the Octave, so nothing gets bumped there by
+    // calendar/index.ts's applyChristmasOctaveSunday (see its own tests) --
+    // a genuinely clean case for this content, unlike most years.
+    const ordo = resolveOrdo('lauds', '2029-12-30');
     const canticle = ordo.parts.find((p) => p.kind === 'canticle' && p.canticleId === 'benedictus');
     expect(canticle?.kind === 'canticle' ? canticle.antiphon?.text.la : undefined).toContain('Dum médium siléntium');
     expect(canticle?.kind === 'canticle' ? canticle.antiphon?.status?.la : undefined).toBe('verified');