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.[1]

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.

It was first suggested by the Neapolitan doctor Aloysius Lilius, and was made official by Pope Gregory XIII, for whom it was named, on February 24, 1582. The official change took place the following October, when Thursday, 4th was followed by Friday, 15th.

The months of the Gregorian calendar year are, in order:

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. The next leap year is 2020.

Not every country started using the new calendar immediately. Spain, Portugal, and Italy started to use the new calendar on Friday, October 15, 1582, following Julian Thursday, October 4, 1582.[2] In Europe's Protestant countries, people feared that the new calendar was an attempt by the Catholic Church to silence their movement. The England and the rest of the British Empire 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).[1][3]

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.[4]

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 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" (西暦, seireki) using western year numbers, is also widely accepted by civilians and to a lesser extent by government agencies.

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.[note 1][5]

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.[5][6]

In the old calendar the year started on March 25. [note 1] This became April 5, 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 April 6. But it was not changed in 1900, so the tax year in the United Kingdom still begins on April 6.

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.