At around the 4th century, the Universal Church agreed to celebrate the birth of Christ on December 25th (29 Kiahk in the Coptic calendar). At that time, and until the 16th century, the Julian calendar (which was introduced by Julius Caesar in the year 46 B.C.) was the standard. In the 16th century, Pope Gregory XIII of Rome established a more accurate solar calendar (based on the sun), which was slightly shorter than the Julian calendar, accumulating to a difference of one full day every 128 years.  They accounted for this difference by pushing the date ahead by 10 days in the month of October 1582. Although this wasn’t adopted around the entire world immediately, the Gregorian calendar is the standard one used today. Some of the Orthodox Churches continued to use the Julian calendar’s designated Christmas, which is currently 13 days behind the Gregorian calendar, placing Christmas on January 7th (29 Kiahk) . So while most of the western Churches, like the Roman Catholic Church, celebrate Christmas on December 25th, we celebrate it on January 7th.