We celebrate Christmas on the 7th of January because it is the date of the Orthodox Church's Christmas. The Orthodox Church follows a different calendar than the traditional Gregorian calendar that is used by most of the world. In the Orthodox calendar, Christmas is celebrated on the 7th of January. This is due to a difference in the calculation of the date of the birth of Jesus Christ, which is celebrated as Christmas. The Orthodox Church uses the Julian calendar, which is 13 days behind the Gregorian calendar.