This article is from the Calendars FAQ, by Claus Tondering email@example.com with numerous contributions by others.
The Gregorian calendar is the one commonly used today. It was proposed
by Aloysius Lilius, a physician from Naples, and adopted by Pope
Gregory XIII in accordance with instructions from the Council of Trent
(1545-1563) to correct for errors in the older Julian Calendar. It was
decreed by Pope Gregory XIII in a papal bull on 24 February 1582. This
bull is named "Inter Gravissimas" after its first two words.
In the Gregorian calendar, the tropical year is approximated as
365 97/400 days = 365.2425 days. Thus it takes approximately 3300
years for the tropical year to shift one day with respect to the
The approximation 365 97/400 is achieved by having 97 leap years
every 400 years.