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The Gregorian calendar is the calendar that is used throughout most of the world. It began to be used from 1582. It replaced the previous Julian calendar because the Julian Calendar had an error: it added a leap year (with an extra day every four years) with no exceptions. The length of the Julian year was exactly 365.25 days (365 days and 6 hours), but the actual time it takes for the Earth to go around the Sun once is closer to 365.2425 days (about 365 days, 5 hours and 49 minutes). This difference is about eleven minutes each year.
This made the seasons get out of track, since the real first day of spring in western Europe (the equinox - day and night the same length) was happening earlier and earlier before the traditional March 21 as the centuries went by. By the 1500s, it was starting around March 11, ten days 'too early' according to the calendar. So what they did was to move the calendar forward ten days in 1582, and at the same time to make sure it did not happen again. To do this, they made an exception to the previous 'leap year rule' (add February 29 every four years). There would be no February 29 for every year that ends in 00 - unless it could be divided by 400. So the year 2000 was a leap year, because it could be divided by 400, but 1700, 1800, and 1900 would be common years, with no February 29.
The months of the Gregorian calendar year are, in order:
- January (31 days)
- February (28 or 29 days)
- March (31 days)
- April (30 days)
- May (31 days)
- June (30 days)
- July (31 days)
- August (31 days)
- September (30 days)
- October (31 days)
- November (30 days)
- December (31 days)
If February has 28 days, then the year is 365 days long. If February has 29 days, then the year is called a leap year and it is 366 days long. A leap year usually happens once every four years. Some examples of leap years are 2004, 2008, and 2012.
Not every country started using the new calendar immediately. Spain, Portugal, the Polish-Lithuanian Commonwealth, and most of Italy started to use the new calendar on Friday, 15 October 1582, following Julian Thursday, October 4 1582. The Spanish and Portuguese colonies adopted the calendar later due to the slowness of communication in those days. France adopted the new calendar on Monday, 20 December 1582, following Sunday, December 9 1582.
The Protestant Dutch provinces of Holland and Zeeland also adopted it in December of that year, but most non-Catholic countries objected to because they said the new calendar was a Catholic invention.
The Kingdom of Great Britain and the rest of the British Empire (including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752; by which time it was necessary to correct by eleven days (Wednesday, September 2 1752 being followed by Thursday, September 14 1752).
In Alaska, Friday, October 6, 1867 was followed by Friday, October 18 1867. The day was still called Friday because when the US bought Alaska from Russia, the International Date Line was changed when the Gregorian calendar was adopted.
In the USSR October Revolution of 1917 was celebrated in November. In 1917 the Russian Empire still used the old Julian Calendar. Changing the calendar meant 365 days after the revolution started was now in November 1918.
In 1923 most Eastern Orthodox Churches changed to the Gregorian calendar. Christmas Day is the same as the Catholic and Protestant churches, but the date of Easter continues to be worked out differently.
The Russian Orthodox Church did not want this change, so Russian Christmas Day is about two weeks after the rest of Europe.
Japan adopted the Gregorian calendar's way of working out leap years on 1 January 1873, but the months have numbers instead of names. Japan also starts year one with each new reign, but use reign names not the name an emperor might be best known by in the west. For example the reign names Meiji year 1=1868, Taisho 1=1912, Showa (Emperor Hirohito) 1=1926, Heisei (Emperor Akihito) 1=1989, and so on. The "Western calendar" (西暦, seireki) using western year numbers, is also widely accepted by civilians and to a lesser extent by government agencies.
The Old Calendar today
Old Style and New Style dates
Some old dates in Britain used to be shown with two years. For example King Charles I died on 30 January 1649. This is because Britain did not start a new year until March 25th, so for a few months it was one year in Britain and the next year in Europe. The letter OS (for Old Style) and NS (for New Style) are also used sometimes. Charles I died 30 January 1648 (OS), but the way we work out dates today we would say 10 February 1649.  The difference is important working out how old somebody was, or where to look if researching old documents.
In the old calendar the year started on March 25.  This became 5 April, and was used as the first day of the year for working out taxes and rents. Taxes and rents went on using the old way of working out leap years so in 1800 the year started on 6 April. But it was not changed in 1900, so the tax year in the United Kingdom still begins 6 April.
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People sometimes use the term N.S. or New Style to mean the Gregorian calendar, with Old Style (or O.S.) meaning the Julian calendar.
- Toke Nørby. The Perpetual Calendar: What about France?
- House of Commons Journal Volume 8, 9 June, 1660 Regicides.
- (24 March 1648 for example was followed by 25 March 1649)