Gregorian calendar

The Gregorian calendar is the calendar that is used throughout most of the world. It began being used in 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 with the calendar. For example, as the centuries went by, the equinox (when day and night are the same length) was happening earlier and earlier than its traditional date, March 21. It was the calendar that was wrong, not the earth's orbit. By the 1500s, it was starting around March 11, ten days 'too early' according to the calendar. To farmers, this did not matter because they worked to the seasons rather than to the calendar. But it mattered a lot to the Christian church because the date of Easter is calculated from the equinox being on 21 March. So Pope Gregory XIII declared that the calendar must skip ten days in 1582.
To make sure that the mistake did not happen again, they revised the 'leap year rule'. Until then, every fourth year had February 29, without exception. The change was that 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 official change took place the following October, when Thursday, 4th was followed by Friday, 15th.
Months
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. The most recent leap year was 2020, and the next leap year is 2024.
Adoption
The Pope's instruction only applied to the Catholic Church, countries made their own laws. But Europe's Catholic countries decided to follow his lead. Spain, Portugal, and Italy started to use the new calendar on Friday, October 15, 1582, following Julian Thursday, October 4, 1582. In Europe's Protestant countries, people feared that the new calendar was an attempt by the Catholic Church to silence their movement. Since the Great Schism, Europe's Eastern Orthodox churches also ignored the Pope's decision and many continue to use the Julian calendar even today.
British Empire
England and the rest of the British Empire (including what is now the US and Canada) adopted the Gregorian calendar in 1752. Because 1600 was a leap year in both calendars but the Julian calendar had a leap year in 1700 but the Gregorian calendar did not, this meant that eleven days needed to be skipped. Wednesday, September 2, 1752 was followed by Thursday, September 14, 1752.
Russia
In the USSR, the 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.
Churches
In 1923 some 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, as well as some other Eastern Orthodox Churches such as Georgian and Serbian, did not want this change, so Russian Christmas Day is about two weeks after the rest of Europe.
Japan
Japan adopted the Gregorian calendar's way of working out leap years on January 1, 1873, but the months have numbers instead of names. Japan also starts year one with each new reign, but uses 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" (Xi Li , seireki) using western year numbers, is also widely accepted by civilians and to a lesser extent by government agencies.
Old Calendar in Britain
Old Style and New Style dates
Some old dates in Britain were written and documented with two different years. This is because Britain did not start a new year until March 25, so for a few months it was one year in Britain and the next year in other countries.
The letters OS (for Old Style) and NS (for New Style) were used to help determine the year being used. For example, King Charles I died on January 30, 1649. In "Old Style" it is correct to say that Charles I died January 30, 1648 (OS). Using "New Style", as we determine dates now, the correct date and year would be February 9, 1649.
British Tax
In the old calendar, the year started on March 25, this was the Quarter Day when rents and taxes fell due. This became April 5 which is why the tax year in the United Kingdom begins on April 6.
Timeline
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.