code: public final void setWeekDay(WeekDay weekDay) {
  
          SortedSet<WeekDay> wds = new TreeSet<>();
          if (null != weekDay) {
              wds.add(weekDay);
          }
          setWeekDays(wds);
  
      }
docs: Set the week day the events should occur.
  @param weekDay the week day to set.