I posted an article (reposted, actually) three years ago which sheds light on the origin of December 25th being selected for the Christ Mass. Some fringe "evangelicals" like to say it was a pagan holiday which Christians adapted to have their own celebration at the same time their previous pagan holiday was, but this simply is not true. Please look at the article I posted back in 2007:
I believe this is a fair and objective view of how the Christ Mass became to be celebrated on December 25th.
Glory to God in the highest! Peace on earth to men of goodwill!