Gregorian calendar

The Gregorian calendar is the most widely used calendar in the world. A modification of the Julian calendar, it was first proposed by the Calabrian doctor Aloysius Lilius, and was decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 via the papal bull Inter gravissimas issued from his seat (Villa Mondragone). Years in the calendar are numbered from the traditional birth year of Jesus, which has been labeled the "anno Domini" (AD) era, and is sometimes labeled the "common era" or the "Christian Era" (CE).

The Gregorian Calendar was devised both because the mean Julian Calendar year was slightly too long, causing the vernal equinox to slowly drift backwards in the calendar year, and because the lunar calendar used to compute the date of Easter had grown conspicuously in error as well.

The Gregorian calendar system dealt with these problems by dropping a certain number of days to bring the calendar back into synchronization with the seasons, and then slightly shortening the average number of days in a calendar year, by omitting three Julian leap-days every 400 years. The days omitted are in century years which are not divisible by 400 (specifically: 29 February 1700, 1800, 1900; 2100, 2200, 2300; 2500, 2600, 2700; 2900, etc.).