Adoption of the Gregorian calendar

Calendars in civil use around the world. (Religious use often differs)
  Adopted the Gregorian calendar.
  Uses a modified Gregorian calendar.
  Uses other calendar alongside the Gregorian calendar.
  Not adopted the Gregorian calendar.
Lunario Novo, Secondo la Nuova Riforma della Correttione del l'Anno Riformato da N.S. Gregorio XIII, printed in Rome by Vincenzo Accolti in 1582, one of the first printed editions of the new calendar.

The adoption of the Gregorian Calendar was an event in the modern history of most nations and societies, marking a change from their traditional (or old style) dating system to the modern (or new style) dating system that is widely used around the world today. Some countries adopted the new calendar from 1582, some did not do so before the early twentieth century, and others did so at various dates between; however a number continue to use a different civil calendar. For many the new style calendar is only used for civil purposes and the old style calendar remains used in religious contexts. Today, the Gregorian calendar is the world's most widely used civil calendar.[1][2][3] During – and for some time after – the change between systems, it has been common to use the terms Old Style and New Style when giving dates, to indicate which calendar was used to reckon them.

The Gregorian calendar was decreed in 1582 by the papal bull Inter gravissimas by Pope Gregory XIII, to correct the erroneous assumption in the then-current Julian calendar that a year lasts 365.25 days, when in reality it is about 365.2422 days. (For a more detailed explanation, see the Gregorian calendar article). 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 and the Papal States. The changes he was proposing were changes to the civil calendar, over which he had no formal authority. They required adoption by the civil authorities in each country to have legal effect.

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

Adoption in Catholic countries

Catholic states such as France, the Italian principalities, Poland, Spain (along with her European and overseas possessions), Portugal and the Catholic states of the Holy Roman Empire were first to change to the Gregorian calendar. Thursday, 4 October 1582 was followed by Friday, 15 October 1582, with ten days skipped. Countries that did not change until the 18th century had by then observed an additional leap year (1700), necessitating the dropping of eleven days. Some countries did not change until the 19th or 20th century, necessitating one or two further days to be omitted from the calendar.

Philip II of Spain decreed the change from the Julian to the Gregorian calendar,[4] which affected much of Roman Catholic Europe, as Philip was at the time ruler over Spain and Portugal as well as much of Italy. In these territories, as well as in the Polish–Lithuanian Commonwealth (ruled by Anna Jagiellon) and in the Papal States, the new calendar was implemented on the date specified by the bull, with Julian Thursday, 4 October 1582, being followed by Gregorian Friday, 15 October 1582; the Spanish and Portuguese colonies followed somewhat later de facto because of delay in communication.[5]

Other Catholic countries soon followed. France adopted the new calendar with Sunday, 9 December 1582, being followed by Monday, 20 December 1582.[6] The Dutch provinces of Brabant and Zeeland, and the States General adopted it on 25 December of that year; the provinces forming the Southern Netherlands (modern Belgium) except the Duchy of Brabant adopted it on 1 January 1583; the province of Holland adopted it on 12 January 1583.[7] The seven Catholic Swiss cantons adopted the new calendar in January 1684 while Geneva and several Protestant cantons adopted it in January 1701 or at other dates throughout the 18th century. The two Swiss communes of Schiers and Grüsch were the last areas of Western and Central Europe to switch to the Gregorian calendar, in 1812.[8]