Gregorian Calendar Reform

Timeline

1. 1582Rome · ItalyBuilding on designs by Jesuit astronomer Christopher Clavius and calendar scholar Aloysius Lilius, Pope Gregory XIII issued the Latin papal bull Inter gravissimas, formally replacing the Julian calendar with the Gregorian calendar. The bull revised the leap-year rule (years divisible by 100 but not by 400 lose the leap day) and ordered the calendar to jump forward 10 days in one step, correcting errors accumulated since the Council of Nicaea in 325 CE.
2. 1582Catholic countries of Western Europe · Western EuropeCatholic regions of Italy, Spain, Portugal, and the Polish-Lithuanian Commonwealth were the first to implement the new calendar: the day after Julian October 4, 1582 (Thursday) jumped directly to Gregorian October 15 (Friday), deleting 10 consecutive days to complete the switch. This was the first practical operation of the Gregorian calendar worldwide.
3. 1582–1583France and Catholic regions of Central Europe · Central EuropeFrance completed its calendar switch between December 9 and 20, 1582 (Julian), skipping 10 days, becoming an early major adopter after the first group of countries. In 1583, Austria, Bavaria, and some Catholic princes of the Holy Roman Empire successively implemented the new calendar, marking the reform's full spread across the Catholic world. Thereafter the Gregorian calendar gradually extended to Protestant countries, Orthodox countries, and regions of Asia and Africa, ultimately becoming the globally accepted civil calendar standard in the early 20th century.