After all, the root of "holiday" is "holy day," and as someone who does not believe in deities or holy beings in general, I technically shouldn't be celebrating them. But in the light of the ridiculousness known as being politically correct and eliminating Christmas parties and replacing them with generic "holiday" and "winter" parties if you're lucky or nothing at all if you're not, I propose something I've done since before I cared about atheism (or really knew what it was), and that's celebrating everything.
Celebrating a holiday is not forcing your religion onto anyone as long as the option is provided, and it's not so much telling your guests what to believe in so much as teaching them about your traditions, which I find fascinating. For example, I went to a friend's party last night and got to watch the Hanukkah candle lighting ceremony and eat latkes with applesauce (which was most delicious considering it counteracted the bite of the onions) and mess around with dreidels, among other things, and it was so much fun, even though that's not what I celebrate.
The problem, I think, is that people are so intolerant of others and fail to consider that different traditions are just that, different, and not necessarily wrong. Granted, there are some beliefs that I consider wrong, such as forced gender discrimination and the denial of basic reproductive health rights, but in terms of things like whether you put up a Christmas tree or a menorah or nothing at all, there is no right or wrong.
My solution, as I said before, is to celebrate everything. Make it like elementary school, when we had those cute little worksheets that we got to color in and the little games that we played so we could learn what goes on during Christmas, Hanukkah, Kwanzaa, Ramadan, Diwali, and so on. Not only was it fun, it was educational. It's not like our teachers were forcing anything on to us, and if someone decided "Hey, this is pretty cool," it's not the end of the world if they decide to switch religions.
If anything, it would be detrimental not to have these sorts of mini-celebrations at a young age because it is denying children, who are (and I hate to be cliché but I have to be) the future of our society, basic societal knowledge. For starters, we are all different and we celebrate different things but that's ok, and denying knowledge on any level is wrong. These people that are so concerned about being politically correct that they are in fact insulting everyone by preventing them from carrying on their own customs and cultures. If someone individually doesn't want to partake in anything, that's fine, but don't force people to not partake in anything, because that's just as bad as forcing them to partake in something.
Bring on the latkes.