I do indeed celebrate Christmas, though obviously as a secular holiday. So as the mood takes me, Christmas tree and decorations go up, and I play Christmas albums and movies all the rest.
According to the Religion Stylebook an atheist is, "A person who does not believe in God or other supernatural forces. Some people make a distinction between “weak atheism” (the idea that evidence doesn’t support a belief in God) and “strong atheism” (being convinced that God does not exist)."