Actually I've always celebrated Christmas on the 7th of Jan. Every single year I wanted to look-up why Orthodox and Copts (like myself) celebrate Christmas on 7th of Jan while Western Christians celebrate it on the 25th.
So I found the long answer and I'll posting the link at the bottom, the summary is:
- Early Church fathers tried to estimate Jesus' birthday, by second century AD dates like May 20, April 18, April 19, May 28, January 2, November 17, November 20, March 21 and March 25 were all candidates.
- Modern scholarship estimates the year of Jesus' birth to be between 7 to 4 BC.
- Although the Gospel narratives offer no indication as to the date, they do seem to indicate it was not in the winter.
- By 273 AD, Western Christians had decided on December 25 to celebrate the birth of Jesus. The December date for the holiday probably arose from a desire to provide an alternative to the Roman "birthday of the unconquered sun" which was celebrated on the 25th of Dec.
- Some Orthodox Churches (including the Coptic one I believe) still use the Julian calendar ("old calendar") for their religious calendar. The Julian calendar is the predecessor to the Gregorian calendar ("new calendar") that is now the civil calendar of the western world. The Julian calendar is 13 days different from the Gregorian, so December 25 on the Julian calendar occurs on January 7 on the Gregorian calendar
This was news to me! :) I always thought the the difference between the two dates was because of a certain fundamental disagreement between the Eastern and Western Christan Churches! Seems it's just a technicality depending on which calendar is in use!
For the full article it's here: http://www.religionfacts.com/christianity/holidays/christmas.htm