On October 4, 1582, the following day should have been October 5, but instead, it was October 15.
Where did those 10 days go?
The answer lies in the history of calendars. For many centuries, the Christian world used the Julian calendar, an improvement over the old Roman republican calendar. However, the Julian calendar was still 11 minutes and 14 seconds longer than the tropical year (the time it takes the Sun to return to the same position as seen from Earth). This small difference caused the calendar to drift about one day every 314 years.
One significant problem with this drift was determining the date of Easter. The Council of Nicaea in 325 decided that Easter should be on the first Sunday after the first full moon following the vernal equinox, which was on March 21 at that time. As the calendar drifted, it became harder to determine the correct date for Easter. Despite various proposals for reform, the Julian calendar continued to be used, with its flaws, for many more centuries.
In the 1560s, the Council of Trent called for the pope to fix the calendar problem. After years of study and consultation, Pope Gregory XIII signed a decree in February 1582 to introduce a new calendar, known as the Gregorian calendar. This new calendar was based on suggestions from the Italian scientist Luigi Lilio, with help from the Jesuit mathematician and astronomer Christopher Clavius.
The most unusual part of implementing the new calendar came in October 1582. To correct the drift and realign the calendar with the equinoxes, 10 days were removed from the calendar.