Add the specified weekendDay to the set of weekend days associated with the weekend‐days transition at January 1, 0001 maintained by this calendar. Create a transition at January 1, 0001 if one does not exist. See {Iterator Invalidation} for information regarding which iterators are affected by the use of this method. The behavior is undefined unless no weekend‐days transitions were added to this calendar via the addWeekendDaysTransition method.