diff --git a/src/hours/vespers.ts b/src/hours/vespers.ts
index e8256a6..d78eeb9 100644
--- a/src/hours/vespers.ts
+++ b/src/hours/vespers.ts
@@ -1,6 +1,7 @@
 import type { HourDefinition, HourPart, ResolvedOrdo, ResolvedPart, ResolvedText } from './types';
 import type { LiturgicalDay, Weekday } from '../calendar/types';
 import { resolveEveningDay } from '../calendar/vespers';
+import { weekdayOf } from '../calendar/weekday';
 import { getDayLabel } from '../calendar/day-label';
 import { getPsalmVerses } from '../psalter';
 import { getOpeningVersicleId } from './opening-versicle';
@@ -70,9 +71,16 @@ function psalmParts(group: VespersGroup, day: LiturgicalDay): ResolvedPart[] {
 /** The 4 weekday-variable psalm groups — no fixed leading/trailing psalm
  * the way Lauds has (Ps 66 / the Laudate psalms); Vespers is just these 4
  * groups in sequence. No per-feast override mechanism yet — see
- * hours/types.ts's 'vespers-psalmody' doc comment. */
-function resolvePsalmody(day: LiturgicalDay): ResolvedPart[] {
-  const wd = vespersAntiphons[day.weekday];
+ * hours/types.ts's 'vespers-psalmody' doc comment. Deliberately keyed off
+ * `weekday` (today's actual calendar weekday), not `day.weekday` — `day`
+ * may be tomorrow's identity when tonight anticipates First Vespers (see
+ * resolveEveningDay), but the psalm cycle has no per-feast override and
+ * must stay on today's place in the fixed 6-day rotation regardless of
+ * whose office governs the evening's propers. `day` is still threaded
+ * into psalmParts for antiphon-fullness resolution, which does care who
+ * governs tonight. */
+function resolvePsalmody(day: LiturgicalDay, weekday: Weekday): ResolvedPart[] {
+  const wd = vespersAntiphons[weekday];
   return wd.groups.flatMap((group) => psalmParts(group, day));
 }
 
@@ -154,12 +162,12 @@ function resolveOffice(day: LiturgicalDay): ResolvedPart[] {
   ];
 }
 
-function resolvePart(part: HourPart, day: LiturgicalDay): ResolvedPart[] {
+function resolvePart(part: HourPart, day: LiturgicalDay, weekday: Weekday): ResolvedPart[] {
   switch (part.kind) {
     case 'opening-versicle':
       return [{ kind: 'versicle', text: resolveCommon(getOpeningVersicleId(day.season, day.winner)) }];
     case 'vespers-psalmody':
-      return resolvePsalmody(day);
+      return resolvePsalmody(day, weekday);
     case 'vespers-office':
       return resolveOffice(day);
     case 'magnificat': {
@@ -216,6 +224,10 @@ export function resolveOrdo(date: string): ResolvedOrdo {
   // vespers.ts) — getDayCollects picks that up for free, no special
   // handling needed here.
   const day = resolveEveningDay(date);
-  const parts = vespersDefinition.parts.flatMap((part) => resolvePart(part, day));
+  // Today's own weekday, independent of the possibly-anticipated `day`
+  // above — see resolvePsalmody's doc comment for why psalm selection
+  // must not follow the evening's borrowed identity.
+  const weekday = weekdayOf(date);
+  const parts = vespersDefinition.parts.flatMap((part) => resolvePart(part, day, weekday));
   return { hourId: 'vespers', date, parts, dayLabel: getDayLabel(day) };
 }
diff --git a/tests/hours/vespers.test.ts b/tests/hours/vespers.test.ts
index 391500b..b7b9576 100644
--- a/tests/hours/vespers.test.ts
+++ b/tests/hours/vespers.test.ts
@@ -235,6 +235,20 @@ describe('resolveOrdo("vespers", ...)', () => {
     }
   });
 
+  it('keeps ferial psalmody on the actual calendar weekday even while the office anticipates tomorrow\'s First Vespers', () => {
+    // 2026-08-25 is a Tuesday, but its evening anticipates St. Zephyrinus's
+    // First Vespers (Aug 26) per the test just above -- the psalm cycle has
+    // no per-feast override (see hours/vespers.ts's resolvePsalmody), so it
+    // must stay on Tuesday's own group [129, 130, 131, 132], not slip to
+    // Wednesday's [134, 135, 136, 137] just because the office borrowed
+    // Wednesday's identity.
+    const ordo = resolveOrdo('vespers', '2026-08-25');
+    const psalmNumbers = ordo.parts
+      .filter((p) => p.kind === 'psalm')
+      .map((p) => (p.kind === 'psalm' ? p.psalmNumber : undefined));
+    expect(psalmNumbers).toEqual([129, 130, 131, 132]);
+  });
+
   it("uses Common-of-Several-Martyrs' real office bundle on Ss. Placid and Companions' own day (Duplex II. classis, keeps own Vespers)", () => {
     // Live-verified against Divinum Officium (Monastic Tridentinum 1617)
     // votive=C3 -- see vespers-hymn-common-of-several-martyrs.yml's header.