In 1582 Pope Gregory XIII issued the bull Inter Gravissimas with which he introduced the modification of the calendar that will take his name, the Gregorian Calendar. Until then, the Julian Calendar had been in force, introduced by Giulio Cesare in 46 BC.
10 days were dropped from the calendar to bring the vernal equinox from March 11 back to March 21. The church had chosen October to avoid skipping any major Christian festivals.