DatePicker: update names of weekdays in month view on init (#16689)

* feature (datepicker): update names of weekdays in month view on init

* feature (datepicker): make _initWeekdays a private method

(cherry picked from commit 19453d4)

Author: tobiasschweizer

src/material/datepicker/month-view.ts

@@ -144,16 +144,6 @@ export class MatMonthView<D> implements AfterContentInit {
       throw createMissingDateImplError('MAT_DATE_FORMATS');
     }
 
-    const firstDayOfWeek = this._dateAdapter.getFirstDayOfWeek();
-    const narrowWeekdays = this._dateAdapter.getDayOfWeekNames('narrow');
-    const longWeekdays = this._dateAdapter.getDayOfWeekNames('long');
-
-    // Rotate the labels for days of the week based on the configured first day of the week.
-    let weekdays = longWeekdays.map((long, i) => {
-      return {long, narrow: narrowWeekdays[i]};
-    });
-    this._weekdays = weekdays.slice(firstDayOfWeek).concat(weekdays.slice(0, firstDayOfWeek));
-
     this._activeDate = this._dateAdapter.today();
   }
 
@@ -252,6 +242,7 @@ export class MatMonthView<D> implements AfterContentInit {
         (DAYS_PER_WEEK + this._dateAdapter.getDayOfWeek(firstOfMonth) -
          this._dateAdapter.getFirstDayOfWeek()) % DAYS_PER_WEEK;
 
+    this._initWeekdays();
     this._createWeekCells();
     this._changeDetectorRef.markForCheck();
   }
@@ -261,6 +252,19 @@ export class MatMonthView<D> implements AfterContentInit {
     this._matCalendarBody._focusActiveCell();
   }
 
+  /** Initializes the weekdays. */
+  private _initWeekdays() {
+    const firstDayOfWeek = this._dateAdapter.getFirstDayOfWeek();
+    const narrowWeekdays = this._dateAdapter.getDayOfWeekNames('narrow');
+    const longWeekdays = this._dateAdapter.getDayOfWeekNames('long');
+
+    // Rotate the labels for days of the week based on the configured first day of the week.
+    let weekdays = longWeekdays.map((long, i) => {
+        return {long, narrow: narrowWeekdays[i]};
+    });
+    this._weekdays = weekdays.slice(firstDayOfWeek).concat(weekdays.slice(0, firstDayOfWeek));
+  }
+
   /** Creates MatCalendarCells for the dates in this month. */
   private _createWeekCells() {
     const daysInMonth = this._dateAdapter.getNumDaysInMonth(this.activeDate);