Atheists should celebrate Christmas however they please, especially if they don’t want to celebrate it at all. That’s part of the beauty of free-thinking: no one needs to tell atheists or agnostics how to celebrate or find meaning. Atheists and agnostics, in almost all cases, enjoy discovering their own meaning.
How atheists should celebrate Christmas
If we ask, and try to define, how atheists should celebrate Christmas, we should also ask how they should celebrate Kwanzaa, Hanukkah, and any other religiously-themed winter celebration you can think of. While we’re at it, we should ask what color bald people should dye their hair, how a barren woman should prepare during pregnancy, and how much people’s thoughts weigh in metric tonnes.
Yes, I’m implying that it’s practically nonsensical to ask, “How should atheists celebrate Christmas?” This is first because it assumes that atheists want to celebrate the winter season at all, and second because it assumes that Christianity’s version of winter celebrations should take some sort of precedence over others in an atheist’s, non-theist’s, or agnostic’s mind. There is no reason whatsoever to assume that atheists care about Christmas, although some do happen to.
Some atheists easily divorce legend from reality and are positively enthralled by not only the tree, the holly-scented candles, and Santa, but by the traditional legends behind manger scenes and Christian carols. All is beautiful and artful, representing cultural tradition to them without representing fact.
Other atheists loathe Christmas
All the secular, pagan, and religious symbols alike. Still, others find a happy medium, choosing, for example, to celebrate the winter solstice when everyone else is celebrating what is, to them, a religious day. They love snow and holly and pine, candles and candy canes and Claus – and they keep Jesus out of it, but they don’t necessarily tell other people to do the same thing, depending, of course, on whether Jesus is occupying government or private property.
When it comes to celebrations, atheists don’t have to follow rules, and likely won’t.
If they want to express dissent about Christian displays on government property, that’s their right and a great celebration of secularism. If they want to sing “Away in a Manger” at a neighbor’s door with their family, that’s fine too. How you celebrate is up to you, and how other people celebrate is up to them: no one needs to tell anyone else how to do it, period.