Return the holiday at the specified index in this calendar. For all index values from 0 to numHolidays() ‐ 1 (inclusive), a unique holiday is returned. The mapping of index to holiday is invalidated when the set of holidays is modified by an invocation of addHoliday, addHolidayIfInRange, intersectBusinessDays, intersectNonBusinessDays, removeAll, removeHoliday, setValidRange, unionBusinessDays, or unionNonBusinessDays. The behavior is undefined unless 0 <= index < numHolidays().