All the Atheists I know celebrate only the non-religious aspects of X-mas. Traditions such as gift giving and tree decorating are from the Pagan celebrations of already established winter holidays, including the Winter Solstice. Christians "adopted" (aka: stole) these traditions because it was easier to convert people to Christianity if they didn't completely take away all the aspects of their already established holidays. People became sheeple and just continue to mindlessly follow the herd.