I thought this bit from commenter Luke was perceptive:
It is not just the faith of a lot of the American people that makes Christmas an important national holiday: it is that Christianity was the faith that enabled our cultural ancestors -- at a terrible price -- to build and bequeath to us the civilization we live in, like none other ever seen. This History -- these Facts -- are what justify the celebration, quite apart from the present state of the faith. We have an obligation to remember, just as we have an obligation to remember Lincoln, and the Civil War, and the Battle Hymn of the Republic. As a culture and a civilization, they are all parts of who we are and where we came from. If nothing else we owe it to our children to celebrate and remember these things, lest they take it all for granted -- the and let it slip away.