Gregorian Calendar

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The Gregorian Calendar, a modification of the previously used [Julian calendar]?, was first proposed by Neapolitan doctor Aloysius Lilius, and adopted by [Pope Gregory XIII]?. The mean year in the Julian Calendar had exactly 365.25 days, but given that the real duration is approximately 365.2422, every thousand years the Julian date will be off by about 7.8 days. The church was interested in having Easter celebrated at the correct time, so it was decided that the calendar would be modified. The fix was to define that years divisible by 100 will be [leap year]?s only if they are divisible by 400 as well. So, in the last millenium, 1200, 1600 and 2000 were leap years, but 1100, 1300, 1400, 1500, 1700, 1800 and 1900 where not. That gives a correction on 7 days. This millenium will have 8 days corrected.

The Gregorian calendar also fixed the first day of the year as January 1, which was already the first day used in Italy, Germany, and other places, but not universally (the British Empire, for example, began the year on March 25). This is why the months named after the Latin numbers 7, 8, 9, and 10 (September, October, November, December) are now the 9th, 10th, 11th, and 12th month, respectively.

When the new calendar was put in use, to correct the error already accumulated in sixteen centuries, a jump of eleven days was made passing from October 4, 1582 directly to October 15, 1582. This created some consternation, and the church was accused of stealing days of peoples lives.

Not all countries accepted the new calendar immediately. The [British Empire]? (including what is now the United States) didn't adopt it until 1752, by which time it was necessary to correct by twelve days (September 2, 1752 being followed by September 14, 1752). Russia did not accept the new calendar until 1918, which has the bizarre consequence that the anniversary of the [October revolution]? now falls in November.