@scottjenson those could of course be animated, and even in some "hardware” perpetual calendars they can be changed around https://en.wikipedia.org/wiki/Perpetual_calendar
But basically it boild down to that it's a design choice based on some 20th-century wall calendar designs; decoupling the “day of the month" number from expressing what day of the week it is (which is kept as a separate data point)
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