The Gregorian calendar is the most widely used civil calendar in the world. It is named after Pope Gregory XIII, who introduced it in October 1582. The calendar spaces leap years to make the average year 365.2425 days long, approximating the 365.2422 day tropical year that is determined by the Earth 's revolution around the Sun. The rule for leap years is as follows:
when did we change from julian to gregorian calendar
October 1582