Some atheists really enjoy Christmas. I think it is great that they enjoy the holiday, but I am somewhat puzzled by the defensiveness that often seems to surrounds it. If celebrating Christmas is something you enjoy, I hope you have a blast celebrating it. There is no need to be so defensive about doing so.
Some of us choose not celebrate Christmas - not because we think other atheists should do the same - but because we do not enjoy the holiday or do not find it worth celebrating. I have seen very little effort on the part of those of us who do not celebrate the holiday to convince the celebrating atheists that they are wrong. This leads me to speculate that the defensiveness with which some atheists approach their personal decision to celebrate Christmas might be more about their own discomfort and less about the reactions they receive from others.
Who Says You Shouldn't Celebrate Christmas?
Some atheists seem to have a bit of a complex when it comes to Christmas. They enjoy it but feel that they must defend their enjoyment against criticism. But is this criticism real or imagined?
Just because most people in the U.S. view Christmas as a religious holiday does not mean an atheist must share this perspective or that an atheist cannot enjoy Christmas. As some atheists seem quite fond of reminding us, this was a pagan celebration long before early Christians appropriated it. That being the case, atheists should feel perfectly free to re-appropriate it and turn it into whatever they'd like.
You do not need to invent justifications for your enjoyment of the holiday (e.g., calling it Solstice or calling yourself a "cultural Christian" like Richard Dawkins does). Just enjoy the occasion if you want to and do not feel like you must defend it, especially if you are not being asked to do so.
What If You'd Rather Ignore Christmas?
If you'd prefer to ignore Christmas, feel free to do that as well. This position will place you in a minority, even among atheists. You may feel like you need to justify it, and you may indeed be criticized for it. I certainly have.
Strangely, the people most likely to criticize you (at least in my experience) are not Christians but other atheists. As noted above, many seem to have some issues when it comes to their celebration of Christmas, and this can indeed come across like they are trying to argue with you. Whether or not you explain why you'd prefer to have nothing to do with Christmas is up to you. I am usually willing to explain why I'd prefer to have nothing to do with Christmas and leave it at that. I see little reason to get defensive about it or try to convince anyone else that they should do things my way.
Christian extremists are going to have their annual "war on Christmas" fundraising ploy regardless of what you or I decide to do. It has been very profitable for them because there are lots of really stupid people out there. My suggestion is that you try to enjoy this war as much as I do. As for Christmas itself, I'd like to see us reach the point where atheists should feel free to celebrate it or ignore it without having to feel defensive about their decision.