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There are two kinds of such calendars: ordered ones, and unordered ones meant as fun games, (or, I suppose, reminders of piety), where you have to search for the next one.
I'm actually not certain of the history of the latter, but it is common for promotional branded advent calendars. The typical example are those single-use calendars given as gifts that have a Christmas theme graphic on the front where you have to find the doors, windows, etc., hiding a chocolate or other treat.
On reading the Wikipedia article for advent calendars, they showed many examples of historical Nativity and Christmas winter scenes with unordered openings, so perhaps the unordered type are more "original."