Owing to liturgical differences, the Churches of the Eastern Roman Empire celebrated Christmas Day on January 6 while those in the Western Roman Empire Churches did so on December 25. In October, 1582, Pope Gregory XIII introduced the Gregorian calendar as a correction to the Julian calendar, which meant Christmas Day was now celebrated on December 25, and “Old Christmas” on January 6, the Feast of the Epiphany. This feast day celebrates the manifestation of the Son of God on earth, and also marks the end of the 12 days of Christmas.