One should be able to specify sequences of days, weeks, months, years, and sets of holidays, but be able to have these sequences change at different points. This is important because, in Real Life, rulers often arbitrarily changed calendars at points, especially in ancient times but as recent as in early modern times, and yet one would want to be able to arbitrarily change the calendar however one wants.
Holidays will be flexible, but not the whole calendar system (except for intercalation and leap years). If you want to do a big switch, e.g. from Julian to Gregorian, then you'll create a separate calendar. Otherwise that's just ridiculous amounts of bloat.
One should also be able to have time that is relative to a given "era", where the era may change, sometimes frequently (frequently with changes in rulers). The reason for this is that such eras were the norm for calendars in East Asia up until pretty recently, all things considered.