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 years only if they are divisible by 400 as well. So, in the last millennium, 1200, 1600 and 2000 were leap years, but 1100, 1300, 1400, 1500, 1700, 1800 and 1900 were not. That gives a correction on 7 days. This millennium 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).
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.
The Gregorian calendar can be extended to dates preceding its official introduction, producing the Proleptic Gregorian Calendar.
The Gregorian calendar's year is divided into 12 months: January, February, March, April, May, June, July, August, September, October, November, and December.
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