I think that atheists are split into four different camps:
- Those that celebrate Christmas as a non-religious holiday
- Those that celebrate an alternate winter holiday such as Winter Solstice
- Those that celebrate "the holidays" without using the term "Christmas"
- Those that don't celebrate any winter holiday at all
How could Christmas be "non-religious"? Well, Christmas is an official federal holiday in the United States. While some consider this to be a violation of the separation of church and state, courts have ruled that "the establishment of Christmas Day as a legal public holiday does not violate the Establishment Clause because it has a valid secular purpose" (Ganulin v. United States, 1999). Therefore, it seems perfectly reasonable that Christmas, as celebrated in 21st century America, has become secularized...yet many atheists are uncomfortable with this arrangement. Hence the explosion in popularity of the Winter Solstice.
The use of the phrase "happy holidays" is also popular with not only atheists but many other diverse groups seeking to acknowledge all fall/winter celebrations, including Christmas, Winter Solstice, Kwanzaa, Hanukkah, Diwali, etc.
For the atheist that doesn't celebrate any winter holiday, though? Bah humbug.