Well, the title is pretty self-explanatory, but just for the sake of clarity, answer in the way that makes sense to you. The tradition doesn't have to be uniquely atheist; it just has to be one that has some personal significance.
If you have none, perhaps there is a tradition you would like to start. Despite the widely varying views on Christmas itself, I think it's a good time to take a break and enjoy something ceremonious (more or less). I think traditions are healthy provided people know when to let go of them.