when do orthodox christians celebrate christmas

Christmas is celebrated on January 7th in some Eastern Orthodox churches, following the Julian calendar. This is because the Orthodox Church uses the Julian calendar, which is 13 days behind the modern Gregorian calendar, the one we use today. So, while the rest of the world celebrates Christmas on December 25th, the Orthodox Church celebrates it 13 days later, on January 7th. This is due to the difference in how the two calendars count time.