code: public void setFirstDayOfWeek(int value)
      {
          if (firstDayOfWeek != value) {
              if (value < SUNDAY || value > SATURDAY) {
                  throw new IllegalArgumentException("Invalid day of week");
              }
              firstDayOfWeek = value;
              areFieldsSet = false;
          }
      }
docs: Sets what the first day of the week is,
  where 1 = {@link #SUNDAY} and 7 = {@link #SATURDAY}.
  @param value the given first day of the week, where 1 = {@link #SUNDAY} and 7 = {@link #SATURDAY}.