Adoption of the Gregorian calendar

The adoption of the Gregorian Calendar has taken place in the history of most cultures and societies around the world, marking a change from one of various traditional (or "old style") dating systems to the contemporary (or "new style") system – the Gregorian Calendar – which is widely used around the world today. Some polities adopted the new calendar in 1582, others not before the early twentieth century, and others at various dates between. A few have yet to do so, but except for these, the Gregorian Calendar is now the universal civil calendar, yet old style calendars remain in use in religious or traditional contexts. During – and for some time after – the transition between systems, it has been common to use the terms "Old Style" and "New Style" in dating to indicate which calendar was used to reckon them.

Pope Gregory XIII instituted the Gregorian Calendar in 1582 by the Papal bull Inter gravissimas to correct an error in the Julian Calendar that was causing an erroneous calculation of the date of Easter. The Julian Calendar had been based on a year of 365.25 days, but this was slightly too long; in reality it is circa 365.2422 days, and so over centuries the Calendar had drifted increasingly out of alignment with the orbit of Earth. According to Gregory's scientific advisors, the Calendar had acquired 10 excess leap days since the First Council of Nicaea, which established the rule for dating Easter in AD 325. Consequently, he ruled, the numbering, i. e. dating, of days must jump by 10 to restore the status quo ante; thus, for example, when the Catholic polities of Europe adopted the Gregorian Calendar, the day after Thursday, 4 October 1582 was dated as Friday, 15 October 1582. Polities that did not change calendars until the 18th century had by then observed 1700 as an additional leap year, necessitating the omission of 11 dates from the reckoning. Some polities did not change until the 19th or 20th centuries, necessitating the omission of 12 or 13 dates.

Although Gregory's reform was instituted in the most solemn of forms available to the Church, the bull had no authority beyond ecclesial institutions and the Papal States. The changes he proposed were to the civil calendar, which was not directly subject to Papal authority. For these changes to be legally effectuated, the civil authority of each polity was needed. The bull became canon law of the Catholic Church in 1582, but Protestant churches, Eastern Orthodox Churches, and a few others did not effectuate it, and therefore the dates on which different Christian denominations celebrated Easter and other holidays diverged.