I just looked at the entry for Christmas on wikipedia. I don't remember it being so thoroughly sectarian as it is now, asserting flatly that the holiday is a commemoration of the birth of Jesus. There is no mention of Mithra at all. And, over on the Mithra entry there is no mention of December 25th being Mithra's birthday.
What's next? Will it be asserted flatly that "goodbye" means "God Be With You" with the implication that anyone who says it is at least a theist if not Christian?