code: public final void showYearMonth(YearMonth yearMonth) {
          requireNonNull(yearMonth);
  
          if (!monthPage.isHidden()) {
              selectedPage.set(getMonthPage());
          } else if (!yearPage.isHidden()) {
              selectedPage.set(getYearPage());
          }
  
          setDate(yearMonth.atDay(1));
      }
docs: Sends the request to the calendar view to display the given year and
  month. The view will switch to the {@link MonthPage} and set the value of
  {@link #dateProperty()} to the first day of the month.
  
  @param yearMonth the year and month to show in the view