That's OK, but celebrate Solstice, Yule, The Great Nothingness, whatever, just don't say you're celebrating Christmas--us Christians will take care of that. And when Cinco de Mayo comes around, feel free to celebrate May Day, Jose Cuervo, Drunken Gringo Day, etc. but don't insult the Mexicans by acting like you belong.
I think it's fair to say that by the early 21st Century a large part of the Christmas celebration has become secular. To be sure many people (to varying degrees) celebrate it as a religious occasion, but in popular culture I believe that Christmas' non-religious aspects have partially overtaken the religious aspects.
That being the case, I will continue to celebrate Christmas in spite of being an unbeliever. Just don't expect to see me in church... :-)