Well, I guess you'd have to somehow replace Christmas Day with something. You could call it 'Atheism Day', celebrating the height of the atheistic winter season.
You may as well keep the Christmas Tree, because, anyway, those Christians nicked that idea from the pagans, right? Atheism Day decorations could be as pictured left to remind the citizens of the glories of atheistic regimes of the 20th century, such as mass murder on a hitherto unprecedented scale.
In the local churches, which by then will have become centres of atheist learning, passages are read out from 'the new Gospels' of The Selfish Gene and The God Delusion and maybe God is Not Great. Then, after a big meal, everyone sits back and enjoys some of the classic science lectures of Professor Steve Jones. Then, adverts for companies say things like, "Discover, this winter season, the magic of Atheism...Buy her something special this Atheist Day."
Oh kids would love it. Love it they would. No Santa stories though, because, as I'm sure Dawkins told his children, he doesn't exist and atheists do not feed their children with myths, be they religious or not. Kids would still love it, because we all love and appreciate 'the magic of atheism'. I don't know why but it just doesn't seem to have the same ring about it.