On what day of the week was the Gregorian calendar first introduced on the globe?

The Gregorian calendar was introduced after October 4, 1582: October 4 was immediately followed by October 15 (and not October 5). Using Zeller’s formula, it is easy to find that October 4, 1582 was Thursday, and October 15, 1582 was Friday. Hence the Gregorian calendar is introduced on Friday (5/15 October 1582).

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