FirstWeekContainsDate
FirstWeekContainsDate is used to determine which week is the first week of the year, based on what day the January, 1 is in that week.
The day in that week can only be 1 (Monday) or 4 (Thursday).
Please see https://en.wikipedia.org/wiki/Week#The_ISO_week_date_system for more information.
type FirstWeekContainsDate = 1 | 4;