Good question. Dr. Alvin Schmidt has written about this very topic in his book, How Christianity Changed the World (Grand Rapids, MI: Zondervan, 2005), 377-378. I can't copy the text here due to copyright, but HERE is the link. Just scroll to pages 377-378. Basically, Dr. Schmidt argues that Christmas was celebrated by the early Christians decades before the worship of the sun god began. In fact, these ancient religions tried to paganize and discredit Christianity. Thus, we do not celebrate Christmas on the same day because we are following other pagan traditions.
[Answered by Pastor HM]
