I don't know what atheists do on Christmas. Maybe they do still treat their kids to Santa Claus, but I suspect that they are not putting Nativity Scenes in their yards just to be cultural.
But in the US, do atheist do anything special on Easter other than take Good Friday off from work when it is offered? I suspect not. I suspect that they acknowledge such holidays they same way I honor Muslim holidays, namely by being respectful to others who do celebrate them.
Why limit your question to atheists? I am not an atheist; I am also not a Christian. I think that qualifies me to answer your question.
I am not a Christian.
I celebrate Christmas and Easter. Christmas is hugely commercialized, so there isn't much I do on this holiday that can really be said to be 'Christian', except that it is one of the only two days of the year I might find myself in a church and actually having a good time. It's an enjoyable atmosphere: folk singing, cheering, drinking cider. I've even brought my guitar and performed a couple songs myselfabout Jesus!
I am not a Christian.
Easter is an easier holiday to avoid as a non-Christian; but I celebrate it nonetheless. A big feast, perhaps some church. The Passion plays around town are always good to watch. And, of course, plenty of Easter-themed posts to participate in right here at EvC.
I am not a Christian.
But I do partake in these Christian holidays. Almost all of my morality is Christian-derived. I know many Christians. I often count myself as a 'practicing' Christian' (not a believing Christian; one whowillingly and enjoyablygoes through the motions).
I am not a Christian.
I find your assumption that only Christians celebrate Christian holidays to be absurd. In fact, I have never met someone who does not celebrate Christmas and Easter, except for folks who were explicitly of a religion that did not contain these holidays (Islam, for example). The atheists I know in person all celebrate these holidays. In fact, I can tell you that based on talks with them that they, like me, hold mostly Christian-derived morals.
All of this deep-rooted Christian culture and yet...
I am not a Christian.
Love your enemies!