diff --git a/src/calendar/date-math.ts b/src/calendar/date-math.ts
index e2b0438..9596635 100644
--- a/src/calendar/date-math.ts
+++ b/src/calendar/date-math.ts
@@ -25,3 +25,8 @@ export function daysBetween(fromIsoDate: string, toIsoDate: string): number {
   const to = new Date(`${toIsoDate}T00:00:00Z`).getTime();
   return Math.round((to - from) / 86_400_000);
 }
+
+/** 0 = Sunday, ..., 6 = Saturday. */
+export function dayOfWeek(isoDate: string): number {
+  return new Date(`${isoDate}T00:00:00Z`).getUTCDay();
+}
diff --git a/src/calendar/day-label.ts b/src/calendar/day-label.ts
index f436377..a9d192e 100644
--- a/src/calendar/day-label.ts
+++ b/src/calendar/day-label.ts
@@ -11,7 +11,7 @@
 // forever — just not built yet.
 import type { LiturgicalDay } from './types';
 import { easterSunday } from './easter';
-import { adventStart } from './temporal';
+import { adventStart, firstSundayStrictlyAfter, sundayOnOrBefore } from './temporal';
 import { addDays, daysBetween, toIsoDate } from './date-math';
 
 function capitalize(text: string): string {
@@ -35,20 +35,6 @@ function ordinal(n: number): string {
   }
 }
 
-function weekdayOf(isoDate: string): number {
-  return new Date(`${isoDate}T00:00:00Z`).getUTCDay(); // 0 = Sunday
-}
-
-/** The first Sunday strictly after `isoDate` (even if `isoDate` is itself a Sunday). */
-function firstSundayStrictlyAfter(isoDate: string): string {
-  const dow = weekdayOf(isoDate);
-  return addDays(isoDate, dow === 0 ? 7 : 7 - dow);
-}
-
-function sundayOnOrBefore(isoDate: string): string {
-  return addDays(isoDate, -weekdayOf(isoDate));
-}
-
 interface OrdinalSeason {
   /** ISO date of the season's own anchor day, for a given calendar year. */
   anchorDate(year: number): string;
diff --git a/src/calendar/temporal.ts b/src/calendar/temporal.ts
index 90294c6..38a028f 100644
--- a/src/calendar/temporal.ts
+++ b/src/calendar/temporal.ts
@@ -13,7 +13,7 @@
 // separate, larger project.
 import type { Season, TemporalCategory, Weekday } from './types';
 import { easterSunday } from './easter';
-import { addDays, daysBetween, toIsoDate } from './date-math';
+import { addDays, daysBetween, dayOfWeek, toIsoDate } from './date-math';
 import fixedDateData from '../data/calendar/fixed-date-calendar.yml';
 import easterOffsetsData from '../data/calendar/easter-offsets.yml';
 import temporalCategoriesData from '../data/calendar/temporal-categories.yml';
@@ -45,11 +45,29 @@ const EPIPHANY = fixedDateFor('epiphany');
 /** The Sunday nearest Nov 30 (St. Andrew's Day) — Advent's real start rule. */
 export function adventStart(year: number): string {
   const nov30 = `${year}-11-30`;
-  const dow = new Date(`${nov30}T00:00:00Z`).getUTCDay(); // 0 = Sunday
+  const dow = dayOfWeek(nov30);
   const delta = dow <= 3 ? -dow : 7 - dow;
   return addDays(nov30, delta);
 }
 
+/**
+ * The Sunday on or before `isoDate` — "which week does this feria belong
+ * to." Shared by calendar/day-label.ts (display) and, eventually, whatever
+ * resolves temporal-propers content ids (calendar/types.ts's `PropersRef`
+ * `source: 'temporal'` branch) — both need the same "which Sunday governs
+ * this feria" answer, just for different purposes, so it lives here rather
+ * than in either consumer.
+ */
+export function sundayOnOrBefore(isoDate: string): string {
+  return addDays(isoDate, -dayOfWeek(isoDate));
+}
+
+/** The first Sunday strictly after `isoDate`, even if `isoDate` is itself a Sunday. */
+export function firstSundayStrictlyAfter(isoDate: string): string {
+  const dow = dayOfWeek(isoDate);
+  return addDays(isoDate, dow === 0 ? 7 : 7 - dow);
+}
+
 function seasonFromEasterOffset(offset: number): Season {
   const dayOverride = easterOffsets.days?.[String(offset)];
   if (dayOverride) {