The BACnet Calendar object stores a list of exceptional dates (public holidays, vacations, exceptional closures) that can be referenced by several Schedule objects to apply differentiated behaviour on those days.

The Calendar object is a centralised list of exception dates. Its Date_List property is an array of either a DateRange (start and end date) or a single Date entry, and it is referenced from a Schedule's Exception_Schedule property: on the matching dates the Schedule applies an alternate programme (for example fully OFF instead of the normal weekly profile). The advantage is one Calendar per building (e.g. national public holidays) with a single yearly update and consistent behaviour across every Schedule.

Update the Calendar before the new year, not in January: most buildings have public holidays in early January that catch out teams who postpone the update.