The Gregorian calendar, also called the Western calendar and the Christian calendar, is internationally the most widely accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582; the decree is known by its opening words, Inter gravissimas. The reformed calendar was adopted later that year by a handful of countries, with other countries adopting it over the following centuries.
The motivation for the Gregorian reform was that the Julian calendar assumes that the time between vernal equinoxes is 365.25 days, when in fact it is presently almost 11 minutes shorter. The discrepancy results in a drift of about three days every 400 years. At the time of Gregory's reform there had already been a drift of 10 days since Roman times, resulting in the spring equinox falling on 11 March instead of the ecclesiastically fixed date of 21 March, and moving steadily earlier in the Julian calendar. Because the spring equinox was tied to the celebration of Easter, the Roman Catholic Church considered this steady movement in the date of the equinox undesirable.
The Gregorian reform contained two parts: a reform of the Julian calendar as used prior to Pope Gregory's time and a reform of the lunar cycle used by the Church, with the Julian calendar, to calculate the date of Easter. The reform was a modification of a proposal made by the Calabrian doctor Aloysius Lilius (or Lilio). Lilius' proposal included reducing the number of leap years in four centuries from 100 to 97, by making 3 out of 4 centurial years common instead of leap years: this part of the proposal had been suggested before by, among others, Pietro Pitati. Lilio also produced an original and practical scheme for adjusting the epacts of the moon when calculating the annual date of Easter, solving a long-standing obstacle to calendar reform.
The Gregorian calendar thus modified the Julian calendar's regular cycle of leap years as follows:
- "Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year."
In addition to the change in the mean length of the calendar year from 365.25 days (365 days 6 hours) to 365.2425 days (365 days 5 hours 49 minutes 12 seconds), a reduction of 10 minutes 48 seconds per year, the Gregorian calendar reform also dealt with the accumulated difference between these lengths. Between AD 325 (when the First Council of Nicaea was held, and the vernal equinox occurred approximately 21 March), and the time of Pope Gregory's bull in 1582, the vernal equinox had moved backward in the calendar, until it was occurring on about 11 March, 10 days earlier. The Gregorian calendar therefore began by skipping 10 calendar days, to restore March 21 as the date of the vernal equinox.