diff --git a/src/calendar/index.ts b/src/calendar/index.ts
index 507f273..ac6b3e4 100644
--- a/src/calendar/index.ts
+++ b/src/calendar/index.ts
@@ -109,6 +109,7 @@ export function resolveDay(isoDate: string): LiturgicalDay {
   winner = applyChristmasOctaveSunday(isoDate, weekday, winner, commemorations);
   winner = applyMovableFeasts(isoDate, winner, commemorations);
   applyEpiphany6Commemoration(isoDate, commemorations);
+  applyAdventFourVigilCommemoration(isoDate, weekday, commemorations);
 
   return { date: isoDate, weekday, season, temporalCategory, winner, commemorations };
 }
@@ -141,6 +142,32 @@ function applyEpiphany6Commemoration(isoDate: string, commemorations: Commemorat
   commemorations.push({ kind: 'temporal', id: 'post-epiphany-6' });
 }
 
+/**
+ * When Advent 4's own Sunday lands on Dec 24 (its latest possible date),
+ * `resolveTemporalId` already gives that day the Vigil of Christmas's own
+ * identity (`vigil-of-christmas` — collect, Lauds antiphons, Matins Gospel
+ * nocturn), matching the reference engine's own rule that the Vigil's rank
+ * governs. Per direct instruction (2026-08), unlike the reference engine's
+ * own `no commemoratio` for this exact day, Advent 4 itself is still
+ * commemorated here — the Sundays of Advent have real standing of their
+ * own, distinct from an ordinary saint quietly ceding to a Vigil. Purely
+ * additive, same shape as applyEpiphany6Commemoration just above: no new
+ * `Commemoration` variant needed, `getDayCollects` already resolves any
+ * `{ kind: 'temporal' }` commemoration's own `${id}-collect` for free.
+ * `advent-4` is hardcoded rather than recomputed because Dec 24 always
+ * falls at or after Advent's own 4th Sunday by construction (it's the
+ * latest date Advent's 4th Sunday can ever land on).
+ */
+function applyAdventFourVigilCommemoration(
+  isoDate: string,
+  weekday: ReturnType<typeof weekdayOf>,
+  commemorations: Commemoration[],
+): void {
+  if (isoDate.slice(5) === '12-24' && weekday === 'sunday') {
+    commemorations.push({ kind: 'temporal', id: 'advent-4' });
+  }
+}
+
 /**
  * Per direct instruction, scoped to exactly Dec 26-30 — every year
  * contains exactly one real Sunday somewhere in that 5-day window.
diff --git a/src/calendar/temporal-id.ts b/src/calendar/temporal-id.ts
index 33166db..b2b564e 100644
--- a/src/calendar/temporal-id.ts
+++ b/src/calendar/temporal-id.ts
@@ -30,7 +30,6 @@
 // confirmed against that source rather than re-derived by hand.
 import { resolveSeason, sundayOnOrBefore, firstSundayStrictlyAfter, adventStart, easterOffsetOf } from './temporal';
 import { daysBetween } from './date-math';
-import { weekdayOf } from './weekday';
 
 const EASTER_OFFSET_IDS: [number, string][] = [
   [-63, 'septuagesima'],
@@ -84,13 +83,16 @@ export function resolveTemporalId(isoDate: string): string {
   // Advent 4's own — see data/propers/temporal/vigil-of-christmas-collect.yml
   // and data/propers/nocturn-readings/vigil-of-christmas.yml, both
   // transcribed directly from the reference engine's Sancti/12-24.txt.
-  // Scoped to non-Sunday only for now: when Dec 24 itself is Advent 4's own
-  // Sunday, the reference engine's own 12-24s.txt keeps Advent 4's psalms
-  // and most of its Matins lessons (only swapping in the Vigil's Gospel
-  // nocturn) rather than fully substituting this id — a genuine content
-  // splice this app doesn't model yet, deliberately left as `advent-4` for
-  // that one specific case pending that design decision.
-  if (isoDate.slice(5) === '12-24' && weekdayOf(isoDate) !== 'sunday') {
+  // Applies even when Dec 24 itself is Advent 4's own Sunday — per direct
+  // instruction (2026-08), the Vigil's own identity (rank/collect/Lauds
+  // antiphons) governs that day too, with Advent 4 folded in as a
+  // commemoration instead (calendar/index.ts's
+  // applyAdventFourVigilCommemoration) rather than the reference engine's
+  // own `no commemoratio` rule. Matins' three-nocturn Sunday psalmody is
+  // unaffected either way — it's driven by `day.weekday`, not by this id
+  // (hours/matins.ts's own `threeNocturns` check), so Advent 4's "Psalmi
+  // Dominica" standing survives this id swap for free.
+  if (isoDate.slice(5) === '12-24') {
     return 'vigil-of-christmas';
   }
 
diff --git a/tests/calendar/vigil-of-christmas-sunday.test.ts b/tests/calendar/vigil-of-christmas-sunday.test.ts
new file mode 100644
index 0000000..9001be6
--- /dev/null
+++ b/tests/calendar/vigil-of-christmas-sunday.test.ts
@@ -0,0 +1,72 @@
+import { describe, expect, it } from 'vitest';
+import { resolveDay } from '../../src/calendar';
+import { resolveOrdo } from '../../src/hours';
+
+// Advent 4's own Sunday lands on Dec 24 (its latest possible date) roughly
+// every few years -- 2023 and 2028 are the two nearest examples. Per direct
+// instruction (2026-08): the Vigil of Christmas's own identity governs that
+// day (matching the reference engine's own rule -- Duplex I classis, never
+// displaced even by a privileged Sunday), with Advent 4 itself commemorated
+// alongside it -- a deliberate departure from the reference engine's own
+// `no commemoratio` for this exact collision, since the Sundays of Advent
+// have real standing of their own. See calendar/index.ts's
+// applyAdventFourVigilCommemoration and calendar/temporal-id.ts's own
+// `vigil-of-christmas` case.
+describe('Advent 4 Sunday landing on Dec 24 (the Vigil of Christmas)', () => {
+  it('2023-12-24: Vigil wins, Advent 4 commemorated', () => {
+    const day = resolveDay('2023-12-24');
+    expect(day.weekday).toBe('sunday');
+    expect(day.winner).toEqual({ kind: 'temporal', id: 'vigil-of-christmas' });
+    expect(day.commemorations).toEqual([{ kind: 'temporal', id: 'advent-4' }]);
+  });
+
+  it('2028-12-24: same shape, a second real occurrence', () => {
+    const day = resolveDay('2028-12-24');
+    expect(day.weekday).toBe('sunday');
+    expect(day.winner).toEqual({ kind: 'temporal', id: 'vigil-of-christmas' });
+    expect(day.commemorations).toEqual([{ kind: 'temporal', id: 'advent-4' }]);
+  });
+
+  it("Lauds carries both collects (Vigil's own, then Advent 4's commemorated) and the Vigil's own proper antiphons/Benedictus antiphon", () => {
+    const lauds = resolveOrdo('lauds', '2023-12-24');
+    const collects = lauds.parts.filter((p) => p.kind === 'prayer');
+    expect(collects).toHaveLength(2);
+    for (const c of collects) {
+      if (c.kind !== 'prayer') continue;
+      expect(c.text.status.la).toBe('verified');
+      expect(c.text.status.en).toBe('verified');
+    }
+    expect(collects[0]!.kind === 'prayer' ? collects[0]!.text.text.la : undefined).toContain(
+      'Deus, qui nos redemptiónis nostræ',
+    );
+    expect(collects[1]!.kind === 'prayer' ? collects[1]!.text.text.la : undefined).toContain(
+      'Excita, quǽsumus, Dómine',
+    );
+
+    const benedictus = lauds.parts.find((p) => p.kind === 'canticle' && p.canticleId === 'benedictus');
+    expect(benedictus?.kind).toBe('canticle');
+    if (benedictus?.kind === 'canticle') {
+      expect(benedictus.antiphon?.text.la).toContain('Oriétur');
+      expect(benedictus.antiphon?.status.la).toBe('verified');
+    }
+  });
+
+  it("Matins Nocturn 3 carries the Vigil's own Gospel (Matt 1:18-21) with St. Jerome's homily, still using the Sunday's fixed psalm table throughout", () => {
+    const matins = resolveOrdo('matins', '2023-12-24');
+    const gospels = matins.parts.filter((p) => p.kind === 'gospel');
+    expect(gospels).toHaveLength(1);
+    const gospel = gospels[0]!;
+    expect(gospel.kind).toBe('gospel');
+    if (gospel.kind === 'gospel') {
+      expect(gospel.text.citation?.la).toBe('Matt 1:18-21');
+      expect(gospel.text.status.la).toBe('verified');
+      expect(gospel.homily?.source).toContain('Jerome');
+      expect(gospel.homily?.text.status.la).toBe('verified');
+    }
+    // Nocturns 1-2 keep the ordinary Sunday psalm table (Ps 20-31),
+    // unaffected by which id won -- hours/matins.ts's threeNocturns/
+    // sundayPsalmNocturn path is keyed by day.weekday, not day.winner.id.
+    const psalmCount = matins.parts.filter((p) => p.kind === 'psalm').length;
+    expect(psalmCount).toBeGreaterThan(0);
+  });
+});