Specific
data class SpecificWeekdays(val weekday: Weekday, val nths: List<NthSelector>, val dayOffset: Int = 0) : WeekdaysSelector(source)
At a specific weekday (s) within a month: Only the nths weekday. E.g. only the second Sunday. Optionally, moreover specify an offset to that. E.g. only the day after the second Sunday.