Pope Gregory XIII
Pope Gregory XIII (Bologna, January 1, 1502 - April 10, 1585) was born as Ugo Buoncompagni. On May 13, 1572 he was crowned Pope Gregorius XIII. On October 5, 1582, he introduced the Gregorian Calendar, the calendar which is used today. The previous Julian Calendar ran ten days behind, and to compensate, the date moved to October 15 after October 4.