I take a tack that's probably just as offensive to Christians: I celebrate Christmas, but I secularize the crap out of it. I figure that if the Christians can co-opt Yule from the pagans, I can co-opt Christmas from the Christians.
So I go for all the trappings: the tree, the lights, the family get-togethers, gift-giving... but absolutely no religion. No baby Jesus, no church services, no hymns, none of it.
And you know what? My version of Christmas is generally legal on government land: as long as all you do is put up an evergreen with pretty coloured lights, you won't have an issue with the FFRF. Where you'll run into trouble is when you try to bring distinctly religious elements into it: angels, nativity scenes, and the like.
So yes: the secular holiday of Christmas is widespread, and celebrating it doesn't make a person Christian... because there's no Christianity in it. But the fact that Christmas has been secularized doesn't give you free reign to sneak religion through the back door in the wall of separation of church and state.