diff --git a/client/src/components/Planner/PlaceInspector.tsx b/client/src/components/Planner/PlaceInspector.tsx
index 47bc5a3e4..3efa4eec7 100644
--- a/client/src/components/Planner/PlaceInspector.tsx
+++ b/client/src/components/Planner/PlaceInspector.tsx
@@ -39,7 +39,7 @@ import { navigationTargetLabel, getNavigationTargets, openNavigationTarget } fro
 import { TRANSPORT_TYPES, getPlaceBookings } from '../../utils/dayMerge'
 import { NavigationMenu } from '../shared/NavigationMenu'
 import { centredMapCardBox } from '../Map/centredMapCard'
-import { resolveOpenNow, resolvePlaceTimeZone, placeWeekdayIndex, type OpeningPeriod } from './placeOpenState'
+import { resolveOpenNow, resolvePlaceTimeZone, getWeekdayIndex, type OpeningPeriod } from './placeOpenState'
 import { convertHoursLine } from './placeHoursFormat'
 import type { EndDayControlProps } from '../Roadtrip/EndDayControl'
 import VisitControls, { type RoadtripStayControl } from '../Roadtrip/VisitControls'
@@ -136,13 +136,6 @@ function usePlaceDetails(googlePlaceId, osmId, language) {
   return details
 }
 
-function getWeekdayIndex(dateStr, timeZone) {
-  // weekdayDescriptions[0] = Monday … [6] = Sunday
-  if (!dateStr) return placeWeekdayIndex(new Date(), timeZone)
-  const jsDay = new Date(dateStr + 'T12:00:00').getDay()
-  return jsDay === 0 ? 6 : jsDay - 1
-}
-
 function formatFileSize(bytes) {
   if (!bytes) return ''
   if (bytes < 1024) return `${bytes} B`
diff --git a/client/src/components/Planner/placeOpenState.test.ts b/client/src/components/Planner/placeOpenState.test.ts
index 6b20168d2..2f6662f5e 100644
--- a/client/src/components/Planner/placeOpenState.test.ts
+++ b/client/src/components/Planner/placeOpenState.test.ts
@@ -1,4 +1,4 @@
-import { resolveOpenNow, resolvePlaceTimeZone, placeWeekdayIndex } from './placeOpenState';
+import { getWeekdayIndex, resolveOpenNow, resolvePlaceTimeZone, placeWeekdayIndex } from './placeOpenState';
 import type { OpeningPeriod } from './placeOpenState';
 
 // Seoul is UTC+9, Paris UTC+2 in summer — far enough apart that a viewer's clock and
@@ -56,6 +56,18 @@ describe('placeWeekdayIndex', () => {
   });
 });
 
+describe('getWeekdayIndex', () => {
+  it('FE-PLANNER-OPENSTATE-023: reads a plan day by its own date, Monday first', () => {
+    expect(getWeekdayIndex('2026-07-27', 'Asia/Seoul')).toBe(0);
+    expect(getWeekdayIndex('2026-08-02', null)).toBe(6);
+  });
+
+  it('FE-PLANNER-OPENSTATE-024: a day without a date reads today in the place timezone', () => {
+    expect(getWeekdayIndex(null, 'UTC')).toBe(placeWeekdayIndex(new Date(), 'UTC'));
+    expect(getWeekdayIndex(undefined, null)).toBe(placeWeekdayIndex(new Date(), null));
+  });
+});
+
 describe('resolveOpenNow', () => {
   it('FE-PLANNER-OPENSTATE-010: judges the periods in the place timezone, not the viewer one', () => {
     // Sunday 20:00 UTC → Monday 05:00 in Seoul. The server said closed.
diff --git a/client/src/components/Planner/placeOpenState.ts b/client/src/components/Planner/placeOpenState.ts
index 764f624de..495fea861 100644
--- a/client/src/components/Planner/placeOpenState.ts
+++ b/client/src/components/Planner/placeOpenState.ts
@@ -51,6 +51,14 @@ export function placeWeekdayIndex(now: Date, timeZone: string | null): number {
   return jsDay === 0 ? 6 : jsDay - 1
 }
 
+/** Monday-based weekday index of a plan day's date; for a day without one, today's in `timeZone`. */
+export function getWeekdayIndex(dateStr: string | null | undefined, timeZone: string | null): number {
+  // weekdayDescriptions[0] = Monday … [6] = Sunday
+  if (!dateStr) return placeWeekdayIndex(new Date(), timeZone)
+  const jsDay = new Date(dateStr + 'T12:00:00').getDay()
+  return jsDay === 0 ? 6 : jsDay - 1
+}
+
 /** Weekday (Sunday = 0), minute of day and calendar date of `now` in `timeZone`. */
 function localParts(now: Date, timeZone: string | null): { day: number; minutes: number; date: string } | null {
   if (!timeZone) return null