Shift from Julian calendar to Gregorian. Easter wasn't at the right date any more due to the Earth's precession, so the Catholic church changed the calendar.

Different countries changed at different times. Quite a bit of Europe did the switch in October of 1582 when Pope Gregory instituted the new calendar, and skipped 10 days. England waited until 1752, by then the Earth had precessed enough that another day had to be skipped.