Pope Gregor VII. reformed, or corrected, the calendar used throughout the world. The correction was due to the deviation of the "true" astronomical time, or year, from the previously used Julian calendar, which had been used since its introduction by Julius Cesar in 46 B.C..
In Julian calendar, the length of the year had been assumed to be 365.25 days, with every fourth year being a leap year, with an extra day (February 29) to bring its legth to 366 days, compared to the 365 days of the "common" year. AS the "true" year uis a bit shorter, there is an error in that calendar, which summed up to days over the centuries.
This error was noted in the 13th century because of the calculation of the Easter date (which is defined to be the sunday after the first Full Moon in spring). Therefore, in 1414, Pope John XIII. decided to reform the calendar; gowever, due to various difficulties, this reform was delayed. Eventually, it was Pope Gregor XIII., who conducted this reform in 1582 with the help of the astronomer Luigi Lilio, after reaching a consent of Catholic rulers, on February 24, 1582.
The original Gregorian reform was to leave out the number of accumulated "error" days, which had summed up to 10, between (Julian) October 4, 1582 and (Gregorian) October 15, 1582. For the future, it was necessary to leave out three leap days in four centuries, so that the years 1700, 1800, and 1900 were common years while 1600 and 2000 were leap years. This rule means that century years are taken as common years and leave out their leap day, unless they are divisable by 400.
The reform was immediately effective thoughout the Catholic world, including Vatican, Italy, Spain, Portugal and Poland on the initial date, France, Netherlands and Luxemburg later the same year, and the Catholic part of Germany the year after, i.e. 1583, Austria in 1584. For the rest of the world, it took considerable time to be taken over: German protestants switched to the Gregorian Calendar on March 1, 1700, England and "Her" colonies in September 1752, and eventually Russia and the Orthodox world in 1900, when they had to leave out a total of 13 days. A detqiled table follows:
Last Julian First Gregorian Places Date Date
1582 Oct 4 1582 Oct 15 Vatican, Italy, Spain, Portugal, Poland 1582 Dec 9 1582 Dec 20 France 1582 Dec 21 1583 Jan 1 Netherlands, Luxembourg 1583 Oct 5 1583 Oct 16 Bavaria 1583 Oct 5 1583 Oct 17 Salzburg, Tirol, Brixen 1583 Nov 3 1583 Nov 14 Aachen, Cologne 1583 Dec 14 1583 Dec 25 Kaernten, Steiermark 1584 Jan 6 1584 Jan 17 Austria 1584 Jan 11 1584 Jan 22 Switzerland 1587 Oct 21 1587 Nov 1 Hungary 1610 Aug 22 1610 Sep 2 Prussia 1700 Feb 18 1700 Mar 1 Germany (protestant part), Norway, 1700 Dec 31 1701 Jan 12 Basel, Bern, Biel Neuchatel, Sargans, Schaffhausen, Thurgau, Zurich 1752 Sep 2 1752 Sep 14 England 1753 Feb 17 1753 Mar 1 Finland, Sweden 1873 Jan 1 Japan 1911 Nov 20 China 1916 Mar 31 1916 Apr 14 Bulgaria 1918 Jan 31 1918 Feb 14 Russia/Soviet Union 1919 Jan 18 1919 Feb 1 Serbia, Romania 1924 Mar 9 1924 Mar 23 Greece 1925 Dec 18 1926 Jan 1 Turkey 1928 Sep 17 1928 Oct 1 Egypt
It may be noted that also the beginning of the year, the New Years Day, had been observed on different days throughout the world: Observations included December 25, March 1, and in England, March 25. This was switched to January 1 in 1500 e.g. throughout France and Germany, in England in 1752, and only in 1700 in Russia.