Gregorian calendar

===Adoption <span class="anchor" id="Timeline"></span>===
{{main|Adoption of the Gregorian calendar}}
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, over which he had no authority. They required adoption by the civil authorities in each country to have legal effect.

The bull {{lang|la|Inter gravissimas}} became the law of the Catholic Church in 1582, but it was not recognised by [[Protestantism|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.

On 29 September 1582, [[Philip II of Spain]] decreed the change from the Julian to the Gregorian calendar.<ref>{{cite book |last=Kamen |first=Henry |date=1998 |title=Philip of Spain |url= https://books.google.com/books?id=wyWourPR5S8C&q=gregorian%20calendar|publisher=Yale University Press |page=248 |isbn=978-0300078008}}</ref> This affected much of Roman Catholic Europe, as Philip was at the time ruler over [[Iberian Union|Spain and Portugal]] as well as [[History of Italy (1559–1814)|much of Italy]]. In these territories, as well as in the [[Polish–Lithuanian Commonwealth]]<ref>{{Cite web|last=Cohen|first=Jennie|title=6 Things You May Not Know About the Gregorian Calendar|url=https://www.history.com/news/6-things-you-may-not-know-about-the-gregorian-calendar|access-date=23 July 2021|website=HISTORY|date=23 August 2018 |language=en}}</ref> (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. The [[Spanish Empire|Spanish]] and [[Portuguese Empire|Portuguese colonies]] followed somewhat later {{lang|la|de facto}} because of delay in communication.<ref>{{cite web| url=http://hdl.loc.gov/loc.wdl/wdl.2837 |title ="Pragmatica" on the Ten Days of the Year |year =1584 | publisher= [[World Digital Library]]}}: the first known South American imprint, produced in 1584 by Antonio Ricardo, of a four-page edict issued by King Philip II of Spain in 1582, decreeing the change from the Julian to the Gregorian calendar. {{Dead link|date=May 2022}}</ref> The other major Catholic power of Western Europe, France, adopted the change a few months later: 9 December was followed by 20 December.<ref>{{cite web|url=https://www.tondering.dk/claus/cal/gregorian.php|publisher=Tondering.dk|title=The Calendar FAQ: The Gregorian Calendar|accessdate=3 May 2022}}</ref>

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.<ref>{{cite web |url=https://www.legislation.gov.uk/apgb/Geo2/24/23/section/3 |title=Calendar (New Style) Act 1750, Section 3|publisher = Parliament of Great Britain | via=National Archives}}</ref>

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

Prior to 1917, Turkey used the lunar [[Islamic calendar]] with the [[Hijri year|Hijri era]] for general purposes and the Julian calendar for fiscal purposes. The start of the fiscal year was eventually fixed at 1 March and the year number was roughly equivalent to the [[Hijri year]] (see [[Rumi calendar]]). As the solar year is longer than the lunar year this originally entailed the use of "escape years" every so often when the number of the fiscal year would jump. From 1 March 1917 the fiscal year became Gregorian, rather than Julian. On 1 January 1926, the use of the Gregorian calendar was extended to include use for general purposes and the number of the year became the same as in most other countries.

====Adoption by country====
{{hatnote|This is a brief summary. For a comprehensive table, see [[List of adoption dates of the Gregorian calendar by country]]}}
{| class="wikitable"
! Year
! Country/-ies/Areas 
|-
| 1582
| [[Spain]], [[Portugal]], [[France]], [[Polish-Lithuanian Commonwealth]], [[Italy]], Catholic [[Low Countries]], [[Luxembourg]], and colonies thereof
|-
| 1584
| [[Kingdom of Bohemia]], some Catholic [[Swiss cantons]]{{efn|name="CH"|In the [[Old Swiss Confederacy]], [[Helvetic Republic]], or [[Switzerland]], adoptions were made between 1584 and 1811. Some Catholic cantons switched in 1584, some Protestant in 1700/1701. For a complete list see [[List of adoption dates of the Gregorian calendar per country]].}}
|-
| 1610
| [[Prussia]]
|-
| 1648
| [[Alsace]]
|-
| 1682
| [[Strasbourg]]
|-
| 1700
| Protestant [[Low Countries]], [[Norway]], [[Denmark]], some Protestant [[Swiss cantons]]{{efn| name="CH"}}
|-
| 1752
| [[Kingdom of Great Britain|Great Britain]], [[Kingdom of Ireland|Ireland]], and the [[British Empire#"First" British Empire (1707–1783)|"First" British Empire (1707–1783)]]
|-
| 1753
| [[Sweden]] and [[Finland]]
|-
| 1873
| [[Japan]]
|-
| 1875
| [[Egypt]]
|-
| 1896
| [[Korea]]
|-
| 1912
| [[China]], [[Albania]]
|-
| 1915
| [[Latvia]], [[Lithuania]]
|-
| 1916
| [[Bulgaria]]
|-
| 1917
| [[Ottoman Empire]]
|-
| 1918
| [[Ukraine]], [[Russia]], [[Estonia]]
|-
| 1919
| [[Romania]], [[Yugoslavia]]{{efn|1919 in the regions comprising the former Kingdoms of Serbia and Montenegro (present-day Kosovo, Montenegro, Serbia and North Macedonia). The western and northern regions of what became Yugoslavia were already using the Gregorian calendar. For example, most of [[Slovenia]] adopted the Gregorian calendar at the same time as [[Austria]] in 1583. Coastal [[Croatia]], which was at the time ruled by [[Venice]], adopted the Gregorian calendar in 1582. Inland [[Croatia]], ruled by the [[House of Habsburg|Habsburgs]], adopted it in 1587 along with Hungary. The Gregorian calendar was used in [[Bosnia and Herzegovina]] since the 16th century by the [[Catholic Church|Catholic]] population and was formally adopted for government use in 1878 following occupation by [[Austria-Hungary]].}}
|-
| 1923
| [[Greece]]
|-
| 1926
| [[Turkey]] ([[common era]] years; Gregorian dates in use since 1917 Ottoman adoption)
|-
| 2016
| [[Saudi Arabia]]
|}