why do greek orthodox celebrate christmas on december 25

Many of the early Christians in the Roman Empire were Jewish and followed the Jewish calendar, which was lunar-based and had 12 months with no leap days. In order to align their calendar with the Roman calendar, they added leap days and days to the month of February, which is how we ended up with 365 days in the year. As for Christmas, the early Christians celebrated it on January 6th, the same day as the traditional pagan festival of the birth of the sun, and the holiday was called "Christ's Mass" or "Christ-Mass."