code: @Nullable public CalendarDay getSelectedDate() {
      List<CalendarDay> dates = adapter.getSelectedDates();
      if (dates.isEmpty()) {
        return null;
      } else {
        return dates.get(dates.size() - 1);
      }
    }
docs: Get the currently selected date, or null if no selection. Depending on the selection mode,
  you might get different results.
  
  <p>For {@link #SELECTION_MODE_SINGLE}, returns the selected date.</p>
  <p>For {@link #SELECTION_MODE_MULTIPLE}, returns the last date selected.</p>
  <p>For {@link #SELECTION_MODE_RANGE}, returns the last date of the range. In most cases, you
  should probably be using {@link #getSelectedDates()}.</p>
  <p>For {@link #SELECTION_MODE_NONE}, returns null.</p>
  
  @return The selected day, or null if no selection. If in multiple selection mode, this
  will return the last date of the list of selected dates.
  @see MaterialCalendarView#getSelectedDates()