code: public void selectRange(final CalendarDay firstDay, final CalendarDay lastDay) {
      if (firstDay == null || lastDay == null) {
        return;
      } else if (firstDay.isAfter(lastDay)) {
        adapter.selectRange(lastDay, firstDay);
        dispatchOnRangeSelected(adapter.getSelectedDates());
      } else {
        adapter.selectRange(firstDay, lastDay);
        dispatchOnRangeSelected(adapter.getSelectedDates());
      }
    }
docs: Select a fresh range of date including first day and last day.
  
  @param firstDay first day of the range to select
  @param lastDay last day of the range to select