diff --git a/apps/client/src/translations/en/translation.json b/apps/client/src/translations/en/translation.json
index 0d962270d..4a59d0a61 100644
--- a/apps/client/src/translations/en/translation.json
+++ b/apps/client/src/translations/en/translation.json
@@ -1415,8 +1415,13 @@
     "title": "Localization",
     "language": "Language",
     "first-day-of-the-week": "First day of the week",
-    "sunday": "Sunday",
     "monday": "Monday",
+    "tuesday": "Tuesday",
+    "wednesday": "Wednesday",
+    "thursday": "Thursday",
+    "friday": "Friday",
+    "saturday": "Saturday",
+    "sunday": "Sunday",
     "first-week-of-the-year": "First week of the year",
     "first-week-contains-first-day": "First week contains first day of the year",
     "first-week-contains-first-thursday": "First week contains first Thursday of the year",
diff --git a/apps/client/src/widgets/type_widgets/options/i18n.tsx b/apps/client/src/widgets/type_widgets/options/i18n.tsx
index 1084a05a8..4e6ec61e8 100644
--- a/apps/client/src/widgets/type_widgets/options/i18n.tsx
+++ b/apps/client/src/widgets/type_widgets/options/i18n.tsx
@@ -73,13 +73,21 @@ function DateSettings() {
     return (
         <>
             <OptionsRow name="first-day-of-week" label={t("i18n.first-day-of-the-week")}>
-                <FormInlineRadioGroup
+                <FormSelect
                     name="first-day-of-week"
+                    currentValue={firstDayOfWeek}
+                    onChange={setFirstDayOfWeek}
+                    keyProperty="value"
+                    titleProperty="label"
                     values={[
-                        { value: "0", label: t("i18n.sunday") },
-                        { value: "1", label: t("i18n.monday") }
+                        { value: "1", label: t("i18n.monday") },
+                        { value: "2", label: t("i18n.tuesday") },
+                        { value: "3", label: t("i18n.wednesday") },
+                        { value: "4", label: t("i18n.thursday") },
+                        { value: "5", label: t("i18n.friday") },
+                        { value: "6", label: t("i18n.saturday") },
+                        { value: "7", label: t("i18n.sunday") },
                     ]}
-                    currentValue={firstDayOfWeek} onChange={setFirstDayOfWeek}
                 />
             </OptionsRow>
 
diff --git a/apps/server/src/services/date_notes.ts b/apps/server/src/services/date_notes.ts
index afec6c882..7b8849b2d 100644
--- a/apps/server/src/services/date_notes.ts
+++ b/apps/server/src/services/date_notes.ts
@@ -16,10 +16,12 @@ import searchService from "../services/search/services/search.js";
 import sql from "./sql.js";
 import { t } from "i18next";
 import { ordinal } from "./i18n.js";
+import isoWeek from "dayjs/plugin/isoWeek.js";
 
 dayjs.extend(isSameOrAfter);
 dayjs.extend(quarterOfYear);
 dayjs.extend(advancedFormat);
+dayjs.extend(isoWeek);
 
 const CALENDAR_ROOT_LABEL = "calendarRoot";
 const YEAR_LABEL = "yearNote";
@@ -295,88 +297,16 @@ function getMonthNote(dateStr: string, _rootNote: BNote | null = null): BNote {
 }
 
 function getWeekStartDate(date: Dayjs): Dayjs {
-    const day = date.day();
-    let diff: number;
-
-    if (optionService.getOption("firstDayOfWeek") === "0") { // Sunday
-        diff = date.date() - day + (day === 0 ? -6 : 1); // adjust when day is sunday
-    } else { // Monday
-        diff = date.date() - day;
-    }
-
-    const startDate = date.clone().date(diff);
-    return startDate;
+    const firstDayISO = parseInt(optionService.getOptionOrNull("firstDayOfWeek") ?? "1", 10);
+    const day = date.isoWeekday();
+    const diff = (day - firstDayISO + 7) % 7;
+    return date.clone().subtract(diff, "day").startOf("day");
 }
 
-// TODO: Duplicated with getWeekNumber in src/public/app/widgets/buttons/calendar.ts
-// Maybe can be merged later in monorepo setup
 function getWeekNumberStr(date: Dayjs): string {
-    const year = date.year();
-    const dayOfWeek = (day: number) =>
-        (day - parseInt(optionService.getOption("firstDayOfWeek")) + 7) % 7;
-
-    // Get first day of the year and adjust to first week start
-    const jan1 = date.clone().year(year).month(0).date(1);
-    const jan1Weekday = jan1.day();
-    const dayOffset = dayOfWeek(jan1Weekday);
-    let firstWeekStart = jan1.clone().subtract(dayOffset, "day");
-
-    // Adjust based on week rule
-    switch (parseInt(optionService.getOption("firstWeekOfYear"))) {
-        case 1: { // ISO 8601: first week contains Thursday
-            const thursday = firstWeekStart.clone().add(3, "day"); // Monday + 3 = Thursday
-            if (thursday.year() < year) {
-                firstWeekStart = firstWeekStart.add(7, "day");
-            }
-            break;
-        }
-        case 2: { // minDaysInFirstWeek rule
-            const daysInFirstWeek = 7 - dayOffset;
-            if (daysInFirstWeek < parseInt(optionService.getOption("minDaysInFirstWeek"))) {
-                firstWeekStart = firstWeekStart.add(7, "day");
-            }
-            break;
-        }
-        // default case 0: week containing Jan 1 → already handled
-    }
-
-    const diffDays = date.startOf("day").diff(firstWeekStart.startOf("day"), "day");
-    const weekNumber = Math.floor(diffDays / 7) + 1;
-
-    // Handle case when date is before first week start → belongs to last week of previous year
-    if (weekNumber <= 0) {
-        return getWeekNumberStr(date.subtract(1, "day"));
-    }
-
-    // Handle case when date belongs to first week of next year
-    const nextYear = year + 1;
-    const jan1Next = date.clone().year(nextYear).month(0).date(1);
-    const jan1WeekdayNext = jan1Next.day();
-    const offsetNext = dayOfWeek(jan1WeekdayNext);
-    let nextYearWeekStart = jan1Next.clone().subtract(offsetNext, "day");
-
-    switch (parseInt(optionService.getOption("firstWeekOfYear"))) {
-        case 1: {
-            const thursday = nextYearWeekStart.clone().add(3, "day");
-            if (thursday.year() < nextYear) {
-                nextYearWeekStart = nextYearWeekStart.add(7, "day");
-            }
-            break;
-        }
-        case 2: {
-            const daysInFirstWeek = 7 - offsetNext;
-            if (daysInFirstWeek < parseInt(optionService.getOption("minDaysInFirstWeek"))) {
-                nextYearWeekStart = nextYearWeekStart.add(7, "day");
-            }
-            break;
-        }
-    }
-
-    if (date.isSameOrAfter(nextYearWeekStart)) {
-        return `${nextYear}-W01`;
-    }
-
-    return `${year}-W${weekNumber.toString().padStart(2, "0")}`;
+    const isoYear = date.isoWeekYear();
+    const isoWeekNum = date.isoWeek();
+    return `${isoYear}-W${isoWeekNum.toString().padStart(2, "0")}`;
 }
 
 function getWeekFirstDayNote(dateStr: string, rootNote: BNote | null = null) {