code: private void labelMonthIndicatorMousePressed(MouseEvent e) {
          // Skip this function if the settings have not been applied.
          if (settings == null) {
              return;
          }
          // If the month menu is disabled, then return.
          if (settings.getEnableMonthMenu() == false) {
              return;
          }
          // Create and show the month popup menu.
          JPopupMenu monthPopupMenu = new JPopupMenu();
          String[] allLocalMonths = settings.getTranslationArrayStandaloneLongMonthNames();
          for (int i = 0; i < allLocalMonths.length; ++i) {
              final String localMonth = allLocalMonths[i];
              final int localMonthZeroBasedIndexTemp = i;
              if (!localMonth.isEmpty()) {
                  monthPopupMenu.add(new JMenuItem(new AbstractAction(localMonth) {
                      int localMonthZeroBasedIndex = localMonthZeroBasedIndexTemp;
  
                      @Override
                      public void actionPerformed(ActionEvent e) {
                          drawCalendar(displayedYearMonth.getYear(),
                              Month.of(localMonthZeroBasedIndex + 1));
                      }
                  }));
              }
          }
          Point menuLocation = getMonthOrYearMenuLocation(labelMonth, monthPopupMenu);
          monthPopupMenu.show(monthAndYearInnerPanel, menuLocation.x, menuLocation.y);
      }
docs: labelMonthIndicatorMousePressed, This event is called any time that the user clicks on the
  month display label in the calendar. This opens a menu that the user can use to select a new
  month in the same year.