Both me and my girlfriend are Atheists, but we are celebrating it with her mom and sister, (her mom and dad split after 35 years of marriage 1 month after me and her moved in together) But her mom is christian, I think; she knows I'm an atheist, I'm vocal about it if asked, and she did, so, she knows, but she isn't really religous either, at all, she only holds a religion to have something to cling onto. And her sister has a bit of brain damage becuase of a lack of oxygen during birth, so I never got the feeling she asks any kind of questions about exsistance of god, or christmas.
tl:dr We are going to hold an atheist christmas, basically.