why orthodox christmas is on 7th of january

The Orthodox Church celebrates Christmas on January 7th because of the different calendars used by the church. The Julian calendar, which was the calendar used by the Roman Empire at the time of Jesus' birth, was adopted by the Orthodox Church and is still used today. The Julian calendar is 13 days behind the Gregorian calendar, which is used in the West. This means that Christmas in the Orthodox Church is celebrated 13 days later than in the West.