When the Gregorian reform was introduced, some Catholic regions moved directly from October 4 to October 15, 1582, correcting the discrepancy accumulated by the calendar.

Different regions adopted the new calendar at different times. The reform also changed the leap-year rules for century years, so it was more than a one-time jump in dates.

Background

Because the date of Easter is a function – the computus – of the date of the spring equinox in the northern hemisphere, the Catholic Church considered the increasing divergence between the canonical date of the equinox and observed reality to be unacceptable. Easter is celebrated on the Sunday after the ecclesiastical full moon on or after 21 March, which was adopted as an approximation to the March equinox. European scholars had been well aware of the calendar drift since the early medieval period.

Bede, writing in the 8th century, showed that the accumulated error in his time was more than three days. In the thirteenth century, Roger Bacon estimated the error at seven or eight days. Dante, writing c. 1300, was aware of the need for calendar reform. An attempt to go forward with such a reform was undertaken by Pope Sixtus IV, who in 1475 invited Regiomontanus to the Vatican for this purpose. However, the project was interrupted by the death of Regiomontanus shortly after his arrival in Rome. The increase of astronomical knowledge and the precision of observations towards the end of the 15th century made the question more pressing.

Numerous publications over the following decades called for a calendar reform, among them two papers sent to the Vatican by the University of Salamanca in 1515 and 1578, but the project was not taken up again until the 1540s, and implemented only under Pope Gregory XIII (r. 1572–1585).

Adoption

Although Gregory's reform was enacted in the most solemn of forms available to the Church, the bull had no authority beyond the Catholic Church (of which he was the supreme religious authority) and the Papal States (which he personally ruled). The changes that he was proposing were changes to the civil calendar, which required adoption by the civil authorities in each country to have legal effect.

The bull Inter gravissimas became the law of the Catholic Church in 1582, but it was not recognised by Protestant churches, Eastern Orthodox churches, Oriental Orthodox churches, and a few others. Consequently, the days on which Easter and related holidays were celebrated by different Christian churches again diverged.

Many Protestant countries initially objected to adopting a Catholic innovation; some Protestants feared the new calendar was part of a plot to return them to the Catholic fold. For example, the British could not bring themselves to adopt the Catholic system explicitly: the Annexe to their Calendar (New Style) Act 1750 established a computation for the date of Easter that achieved the same result as Gregory's rules, without actually referring to him.

Britain and the British Empire (including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752. Sweden followed in 1753.

Description

The Gregorian calendar, like the Julian calendar, is a solar calendar with 12 months of 28–31 days each. The year in both calendars consists of 365 days, with a leap day being added to February in the leap years. The months and length of months in the Gregorian calendar are the same as for the Julian calendar. The only difference is that the Gregorian calendar omits a leap day in three centurial years every 400 years and leaves the leap day unchanged in the fourth.

A leap year normally occurs every four years: the leap day, historically, was inserted by doubling 24 February – there were indeed two days dated 24 February. However, for many years it has been customary to put the extra day at the end of the month of February, adding a 29 February for the leap day. Before the 1969 revision of its General Roman Calendar, the Catholic Church delayed February feasts after the 23rd by one day in leap years; masses celebrated according to the previous calendar still reflect this delay.