From fc097c0619435d0918c40a3879ca8876a6b6f157 Mon Sep 17 00:00:00 2001
From: Seifeddine Largat <largatseif@outlook.com>
Date: Thu, 25 Jun 2026 20:40:37 +0200
Subject: [PATCH 1/2] feat(calendar): fixedWeeks option for stable 6-row month
 grid

Pads short months up to a fixed 6 week-rows (the max any month can occupy)
so the month-view grid height stays constant. Only affects month mode;
extends (never truncates) so multi-month spans are untouched.

Consumed by rzrvi-mono apps/app /calendar2 (useCalendar fixedWeeks: true).
---
 packages/time/src/calendar/date-core.ts | 26 ++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/packages/time/src/calendar/date-core.ts b/packages/time/src/calendar/date-core.ts
index a4dc8092..5db37672 100644
--- a/packages/time/src/calendar/date-core.ts
+++ b/packages/time/src/calendar/date-core.ts
@@ -60,6 +60,14 @@ export interface DateCoreOptions {
   calendar?: Temporal.CalendarLike
   /** Optional range of dates to be used. */
   range?: DateRange
+  /**
+   * When `true`, the month view always spans a fixed 6 week-rows (the maximum
+   * any month can occupy) instead of the natural 4/5/6, padding with leading
+   * days of the following month. Keeps the grid height stable across months.
+   * Only affects the `month` view mode; ignored for week/day/workWeek.
+   * @default false
+   */
+  fixedWeeks?: boolean
   /** Optional date formatter. */
   dateFormatter?: Intl.DateTimeFormat
   /** Optional time formatter. */
@@ -70,9 +78,10 @@ export interface DateCoreOptions {
 
 export interface ParsedDateCoreOptions extends Omit<
   Required<DateCoreOptions>,
-  'range' | 'dateFormatter' | 'timeFormatter' | 'dateTimeFormatter'
+  'range' | 'fixedWeeks' | 'dateFormatter' | 'timeFormatter' | 'dateTimeFormatter'
 > {
   range: ParsedDateRange
+  fixedWeeks?: boolean
 }
 
 export abstract class DateCore {
@@ -194,6 +203,21 @@ export abstract class DateCore {
             7) %
           7
         end = lastDayOfMonth.add({ days: 6 - lastDayOfMonthWeekDay })
+        // Pad short months up to a fixed 6 week-rows so the grid never shifts.
+        // Only extends (never truncates), so multi-month spans (>6 weeks) are
+        // left untouched.
+        if (this.options.fixedWeeks) {
+          const FIXED_WEEKS = 6
+          // start and end are both week-boundary aligned, so the span is a whole
+          // number of weeks; round defensively so a fractional value can never
+          // reach Temporal's integer-only `add`.
+          const spanWeeks = Math.round(
+            (start.until(end, { largestUnit: 'day' }).days + 1) / 7,
+          )
+          if (spanWeeks < FIXED_WEEKS) {
+            end = end.add({ weeks: FIXED_WEEKS - spanWeeks })
+          }
+        }
         break
       }
       case 'week': {

From 5878ada5ce148720998db137b5b0a418024c523f Mon Sep 17 00:00:00 2001
From: Seifeddine Largat <largatseif@outlook.com>
Date: Thu, 25 Jun 2026 20:40:37 +0200
Subject: [PATCH 2/2] feat(calendar): weekStartsOn option to override locale
 week start

---
 packages/time/src/calendar/date-core.ts | 10 +++++++++-
 packages/time/src/utils/weekUtils.ts    |  9 ++++++---
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/packages/time/src/calendar/date-core.ts b/packages/time/src/calendar/date-core.ts
index 5db37672..2bf416f8 100644
--- a/packages/time/src/calendar/date-core.ts
+++ b/packages/time/src/calendar/date-core.ts
@@ -54,6 +54,11 @@ export interface DateCoreOptions {
   viewMode: ViewMode
   /** Optional locale for date formatting. Uses a BCP 47 language tag. */
   locale?: Intl.UnicodeBCP47LocaleIdentifier
+  /**
+   * Optional first day of the week (ISO: 1=Mon … 7=Sun). Overrides the
+   * locale-derived first day across all views when provided.
+   */
+  weekStartsOn?: number
   /** Optional time zone specification. */
   timeZone?: Temporal.TimeZoneLike
   /** Optional calendar system to be used. */
@@ -78,10 +83,12 @@ export interface DateCoreOptions {
 
 export interface ParsedDateCoreOptions extends Omit<
   Required<DateCoreOptions>,
-  'range' | 'fixedWeeks' | 'dateFormatter' | 'timeFormatter' | 'dateTimeFormatter'
+  'range' | 'fixedWeeks' | 'weekStartsOn' | 'dateFormatter' | 'timeFormatter' | 'dateTimeFormatter'
 > {
   range: ParsedDateRange
   fixedWeeks?: boolean
+  /** ISO 1=Mon … 7=Sun, or undefined to derive from the locale. */
+  weekStartsOn?: number
 }
 
 export abstract class DateCore {
@@ -162,6 +169,7 @@ export abstract class DateCore {
     return getFirstDayOfWeek(
       this.store.state.currentPeriod.toString(),
       this.options.locale,
+      this.options.weekStartsOn,
     )
   }
 
diff --git a/packages/time/src/utils/weekUtils.ts b/packages/time/src/utils/weekUtils.ts
index 7bd05cc5..f8cef44d 100644
--- a/packages/time/src/utils/weekUtils.ts
+++ b/packages/time/src/utils/weekUtils.ts
@@ -13,15 +13,18 @@ export function getFirstDayOfMonth(yearMonth: string): Temporal.PlainDate {
 }
 
 /**
- * Get the first day of the week for a given date string and locale
+ * Get the first day of the week for a given date string and locale.
+ *
+ * `weekStartsOn` (ISO: 1=Mon … 7=Sun) overrides the locale-derived first day
+ * when provided; otherwise the locale's own convention is used.
  */
 export function getFirstDayOfWeek(
   dateString: string,
   locale: string,
+  weekStartsOn?: number,
 ): Temporal.PlainDate {
   const date = Temporal.PlainDate.from(dateString)
-  const weekInfo = getWeekInfo(locale)
-  const firstDayOfWeek = weekInfo.firstDay
+  const firstDayOfWeek = weekStartsOn ?? getWeekInfo(locale).firstDay
   const dayOfWeek = date.dayOfWeek
   const daysToSubtract = (dayOfWeek - firstDayOfWeek + 7) % 7