The intellectual argument goes like this:
Jesus' birth is well documented in the Gospels. His family went up to Jerusalem to tithe during a holiday which any Orthodox Jew can tell you falls sometime between April and May. Jesus was born sometime around Easter.
In Bethlehem (which means "House of Bread"), it has never snowed in December, much less April. There were no fir trees in Bethlehem at the time (though there were probably a few cedars and plenty of grape vines).
The Church, in an effort to lure Teutonic pagans to Jesus, adapted the winter solstice holiday, yule log, pine tree, snow and all, and invented December 25 to be Jesus' birthday. Why not invent it for mid-April when he was really born? Well, that's too close to the rites of Spring, and they already had (correctly) nailed that down as Easter.
So all these trappings are a lie, put in place to appease the people who later went on to try to take over the world twice, killing most of the Jews, and gays in Europe in the process.
And then there's the fact that 2,000 years after the arrival of the Prince of Peace, we have more wars, poverty and strife in the world than ever, so even if it was his birthday, I don't see anything to celebrate.
Playboy used to run the incomparable Gahan Wilson's cartoons full-page, full-color. One of my favorites from, I think, the 60's showed a crowd of people dressed in hooded robes worshipping a sign which said "Nothing". A person not dressed for worship is talking to one of the robed ones, and the caption is "Is Nothing sacred?"