Discussion of the Reform of the Calendar under Pope Gregory XIII (1502-85) replaced by the Gregorian Calendar, Rome, October 15, 1582. Note the scholar in right background pointing to the constellation Scorpio. The Gregorian calendar is the solar calendar in use throughout most of the world. It was sponsored by Pope Gregory XIII in 1582 as a corrected version of the Julian calendar. The ordinary year is made to consist of 365 days and a leap year occurs in every year whose number is divisible by four, except those centenary years, such as 1900, whose numbers are not divisible by 400.