The western church celebrates Easter on the first Sunday after the first full moon following the spring equinox. The Gregorian calendar was a reform of the Julian calendar (introduced by Julius Caesar in 45 BC). There has never been an October 5th, 6th, 7th, 8th, 9th, 10th, 11th, 12th, 13th or 14th, 1582, in the West. Pope Gregory’s concern, however, was not with pub quizzes but with establishing the mathematically correct date for the spring equinox, so the date for Easter Sunday would be correct. Because the spring equinox was last Thursday, March 19th.

Source: The Irish Times March 21, 2020 00:22 UTC