diff --git a/apps/web/src/components/Sidebar.snooze.test.ts b/apps/web/src/components/Sidebar.snooze.test.ts
index 16e17e4217eb..ef776e09a815 100644
--- a/apps/web/src/components/Sidebar.snooze.test.ts
+++ b/apps/web/src/components/Sidebar.snooze.test.ts
@@ -10,7 +10,7 @@ function localDate(year: number, month: number, day: number, hour: number, minut
 describe("resolveSnoozePresets", () => {
   it("offers one hour, three hours, evening, tomorrow, and next week in the morning", () => {
     // Wednesday 2026-04-08 10:00 local.
-    const presets = resolveSnoozePresets(localDate(2026, 4, 8, 10), "locale");
+    const presets = resolveSnoozePresets(localDate(2026, 4, 8, 10), "locale", "en-GB");
     expect(presets.map((preset) => preset.id)).toEqual([
       "hour",
       "three-hours",
@@ -33,7 +33,7 @@ describe("resolveSnoozePresets", () => {
   });
 
   it("whenLabel complements the label instead of repeating it", () => {
-    const presets = resolveSnoozePresets(localDate(2026, 4, 8, 10), "locale");
+    const presets = resolveSnoozePresets(localDate(2026, 4, 8, 10), "locale", "en-GB");
     for (const preset of presets) {
       // Day words live in the label column; the time column is time-only
       // (plus a weekday for next week, which names a different day).
@@ -47,16 +47,18 @@ describe("resolveSnoozePresets", () => {
 
   it("drops the evening preset once evening is near or past", () => {
     expect(
-      resolveSnoozePresets(localDate(2026, 4, 8, 17, 30), "locale").map((preset) => preset.id),
+      resolveSnoozePresets(localDate(2026, 4, 8, 17, 30), "locale", "en-GB").map(
+        (preset) => preset.id,
+      ),
     ).toEqual(["hour", "three-hours", "tomorrow", "next-week"]);
     expect(
-      resolveSnoozePresets(localDate(2026, 4, 8, 21), "locale").map((preset) => preset.id),
+      resolveSnoozePresets(localDate(2026, 4, 8, 21), "locale", "en-GB").map((preset) => preset.id),
     ).toEqual(["hour", "three-hours", "tomorrow", "next-week"]);
   });
 
   it("puts next week a full week out when today is Monday", () => {
     // Monday 2026-04-06.
-    const presets = resolveSnoozePresets(localDate(2026, 4, 6, 10), "locale");
+    const presets = resolveSnoozePresets(localDate(2026, 4, 6, 10), "locale", "en-GB");
     const nextWeek = new Date(presets.find((preset) => preset.id === "next-week")!.snoozedUntil);
     expect(nextWeek.getDay()).toBe(1);
     expect(nextWeek.getDate()).toBe(13);
@@ -93,4 +95,16 @@ describe("snoozeWakeDescription", () => {
       "18:00",
     );
   });
+
+  it("uses the client locale for weekday and calendar-date labels", () => {
+    const weekdayWake = localDate(2026, 4, 13, 9);
+    const dateWake = localDate(2026, 4, 20, 9);
+
+    expect(snoozeWakeDescription(weekdayWake.toISOString(), now, "locale", "de-DE")).toContain(
+      weekdayWake.toLocaleDateString("de-DE", { weekday: "short" }),
+    );
+    expect(snoozeWakeDescription(dateWake.toISOString(), now, "locale", "de-DE")).toContain(
+      dateWake.toLocaleDateString("de-DE", { month: "short", day: "numeric" }),
+    );
+  });
 });
diff --git a/apps/web/src/components/Sidebar.snooze.ts b/apps/web/src/components/Sidebar.snooze.ts
index e7b980279a4b..d417ec295edd 100644
--- a/apps/web/src/components/Sidebar.snooze.ts
+++ b/apps/web/src/components/Sidebar.snooze.ts
@@ -5,7 +5,7 @@ import {
   type SnoozePreset,
 } from "@t3tools/client-runtime/state/thread-settled";
 
-import { formatShortTimestamp, parseTimestampDate } from "../timestampFormat";
+import { formatShortTimestamp, getTimestampLocale, parseTimestampDate } from "../timestampFormat";
 
 export { snoozeWakeLabel, type SnoozePreset };
 
@@ -18,8 +18,9 @@ function timeOfDayLabel(date: Date, timestampFormat: TimestampFormat): string {
 export function resolveSnoozePresets(
   now: Date,
   timestampFormat: TimestampFormat,
+  locale: string | undefined = getTimestampLocale(),
 ): ReadonlyArray<SnoozePreset> {
-  return resolveSharedSnoozePresets(now).map((preset) => {
+  return resolveSharedSnoozePresets(now, locale).map((preset) => {
     const wake = parseTimestampDate(preset.snoozedUntil);
     if (wake === null) return preset;
     const time = timeOfDayLabel(wake, timestampFormat);
@@ -27,7 +28,7 @@ export function resolveSnoozePresets(
       ...preset,
       whenLabel:
         preset.id === "next-week"
-          ? `${wake.toLocaleDateString(undefined, { weekday: "short" })} ${time}`
+          ? `${wake.toLocaleDateString(locale, { weekday: "short" })} ${time}`
           : time,
     };
   });
@@ -41,6 +42,7 @@ export function snoozeWakeDescription(
   snoozedUntil: string,
   now: Date,
   timestampFormat: TimestampFormat,
+  locale: string | undefined = getTimestampLocale(),
 ): string {
   const wake = parseTimestampDate(snoozedUntil);
   if (wake === null) return "";
@@ -50,8 +52,8 @@ export function snoozeWakeDescription(
   const dayDelta = Math.floor((wake.getTime() - startOfToday.getTime()) / DAY_MS);
   if (dayDelta === 0) return time;
   if (dayDelta === 1) return `tomorrow ${time}`;
-  const weekday = wake.toLocaleDateString(undefined, { weekday: "short" });
+  const weekday = wake.toLocaleDateString(locale, { weekday: "short" });
   if (dayDelta < 7) return `${weekday} ${time}`;
-  const date = wake.toLocaleDateString(undefined, { month: "short", day: "numeric" });
+  const date = wake.toLocaleDateString(locale, { month: "short", day: "numeric" });
   return `${date}, ${time}`;
 }
diff --git a/apps/web/src/timestampFormat.ts b/apps/web/src/timestampFormat.ts
index 9dd463bb50fa..81dbf2a14182 100644
--- a/apps/web/src/timestampFormat.ts
+++ b/apps/web/src/timestampFormat.ts
@@ -53,6 +53,10 @@ function readHostSystemLocale(): string | null {
 
 const timestampLocale = resolveTimestampLocale(readHostSystemLocale());
 
+export function getTimestampLocale(): string | undefined {
+  return timestampLocale;
+}
+
 const timestampFormatterCache = new Map<string, Intl.DateTimeFormat>();
 
 function getTimestampFormatter(
diff --git a/packages/client-runtime/src/state/threadSettled.ts b/packages/client-runtime/src/state/threadSettled.ts
index f5209a09e499..796dd20163a7 100644
--- a/packages/client-runtime/src/state/threadSettled.ts
+++ b/packages/client-runtime/src/state/threadSettled.ts
@@ -202,14 +202,32 @@ function addSnoozeDays(base: Date, days: number): Date {
   return next;
 }
 
+type LocaleWeekInfo = { readonly firstDay: number };
+type LocaleWithWeekInfo = Intl.Locale & {
+  readonly weekInfo?: LocaleWeekInfo;
+  getWeekInfo?: () => LocaleWeekInfo;
+};
+
+function firstDayOfWeek(locale: string | undefined): number {
+  try {
+    const resolvedLocale = new Intl.Locale(
+      locale ?? Intl.DateTimeFormat().resolvedOptions().locale,
+    ) as LocaleWithWeekInfo;
+    const firstDay = resolvedLocale.getWeekInfo?.().firstDay ?? resolvedLocale.weekInfo?.firstDay;
+    return firstDay === undefined ? 1 : firstDay % 7;
+  } catch {
+    return 1;
+  }
+}
+
 /**
  * Shared "snooze until" choices for every client. "This evening" only
  * appears while it is meaningfully before evening; after that the calendar
- * choices start at "Tomorrow". Calendar presets that land on the same
- * instant collapse: on Sundays "Tomorrow" and "Next week" are both Monday
- * morning, so only "Tomorrow" is offered.
+ * choices start at "Tomorrow". "Next week" follows the locale's first
+ * weekday. Calendar presets that land on the same instant collapse, so only
+ * "Tomorrow" is offered when tomorrow starts the next locale week.
  */
-export function resolveSnoozePresets(now: Date): ReadonlyArray<SnoozePreset> {
+export function resolveSnoozePresets(now: Date, locale?: string): ReadonlyArray<SnoozePreset> {
   const inAnHour = new Date(now.getTime() + HOUR_MS);
   const inThreeHours = new Date(now.getTime() + 3 * HOUR_MS);
   const presets: SnoozePreset[] = [
@@ -245,8 +263,9 @@ export function resolveSnoozePresets(now: Date): ReadonlyArray<SnoozePreset> {
     snoozedUntil: tomorrow.toISOString(),
   });
 
-  const daysUntilMonday = (1 - now.getDay() + 7) % 7 || 7;
-  const nextWeek = snoozeAtHour(addSnoozeDays(now, daysUntilMonday), MORNING_HOUR);
+  const weekStart = firstDayOfWeek(locale);
+  const daysUntilNextWeek = (weekStart - now.getDay() + 7) % 7 || 7;
+  const nextWeek = snoozeAtHour(addSnoozeDays(now, daysUntilNextWeek), MORNING_HOUR);
   if (nextWeek.getTime() !== tomorrow.getTime()) {
     presets.push({
       id: "next-week",
diff --git a/packages/client-runtime/src/state/threadSnoozed.test.ts b/packages/client-runtime/src/state/threadSnoozed.test.ts
index 8a62103950bf..fde9a5ab7261 100644
--- a/packages/client-runtime/src/state/threadSnoozed.test.ts
+++ b/packages/client-runtime/src/state/threadSnoozed.test.ts
@@ -349,18 +349,23 @@ describe("resolveSnoozePresets", () => {
     ]);
   });
 
-  it("puts next week on the following Monday", () => {
-    const nextWeek = new Date(
-      resolveSnoozePresets(localDate(2026, 4, 6, 10)).find((preset) => preset.id === "next-week")!
-        .snoozedUntil,
+  it("puts next week on the locale's first weekday", () => {
+    const now = localDate(2026, 4, 6, 10);
+    const sundayStart = new Date(
+      resolveSnoozePresets(now, "en-US").find((preset) => preset.id === "next-week")!.snoozedUntil,
     );
-    expect(nextWeek.getDay()).toBe(1);
-    expect(nextWeek.getDate()).toBe(13);
+    const mondayStart = new Date(
+      resolveSnoozePresets(now, "en-GB").find((preset) => preset.id === "next-week")!.snoozedUntil,
+    );
+
+    expect(sundayStart.getDay()).toBe(0);
+    expect(sundayStart.getDate()).toBe(12);
+    expect(mondayStart.getDay()).toBe(1);
+    expect(mondayStart.getDate()).toBe(13);
   });
 
-  it("drops next week on Sundays, when it lands on the same Monday as tomorrow", () => {
-    // Sunday 2026-08-30 07:01: "Tomorrow" and "Next week" are both Monday 9:00.
-    const presets = resolveSnoozePresets(localDate(2026, 8, 30, 7, 1));
+  it("drops next week when it lands on the same day as tomorrow", () => {
+    const presets = resolveSnoozePresets(localDate(2026, 8, 30, 7, 1), "en-GB");
     expect(presets.map((preset) => preset.id)).toEqual([
       "hour",
       "three-hours",