Pope Gregory XIII

Name at birth: Ugo Buoncompagni

Gregory XIII was the 16th century Catholic pope who changed the European calendar to what it is today. A legal scholar from Bologna, he settled in Rome in 1539 and held offices under popes Julius III, Pius IV and Pius V during his career. He attended the Council of Trent as a papal deputy (1559-63) and served as a legate to King Philip II in Spain (1564-66) before returning to Rome as a papal secretary and advisor. Elected pope on 13 May 1572, he continued war with the Turks and established learning centers to fight the rise of Protestantism. Gregory is sometimes criticized because he celebrated the slaughter of French protestants (St. Bartholomew's Massacre) and tried to undermine Elizabeth I in England, but he's mostly famous for correcting the calendar. By papal decree most Catholic countries went from 15 October 1582 to 4 October 1582 and began following a calendar year that was 365 days with an extra day every four years (leap years). In time other countries followed suit and now most of the world operates on the Gregorian calendar.