"Christ the Savior is born..."
"In Bethlehem in Jewry the blessed babe was born..."
"Hark the herald angels sing/Glory to the newborn king..."
So how do Rob and I--two atheists--celebrate it?
Christmas is both my and Rob's favorite holiday. We both go crazy for decorating the tree, for watching a very specific series of Christmas movies (Nightmare Before Christmas, Emmet Otter, Love Actually, Christmas Vacation...if you were interested), for listening to Bing Crosby, and now for passing along our traditions to Amelia. We buy each other gifts, we fill stockings, we will put cookies out for Santa Claus. Though Rob could take or leave them, I love singing Christmas carols and have even (voluntarily and enjoyably) attended Christmas mass with my in-laws.
Neither Rob nor I see any of this as incongruous with our shared atheism.
Much is made in the United States of the fact that Christmas is an increasingly secular holiday celebrated even by the heathenest of heathens like us. Each year the voices bloviating about the so-called "war on Christmas" grow louder and more obnoxious, despite the fact that you are far more likely to encounter messages of "keep Christ in Christmas" than "let's just call it 'mas'." These people are the same that claim religious discrimination because schoolchildren don't sing songs about Jesus in public schools despite the fact that they are perfectly welcome to sing them at home or at church.
The fact is, there is just as many pagan and secular elements to Christmas as there are Christian ones. It is common knowledge that Christmas gains many of its traditions from Pagan celebrations, including Saturnalia which took place in December and was somewhat of a drunken free-for-all. The Christmas Tree and gift-giving are both borrowed from non-Christian celebrations. The Bible makes no direct reference to when Jesus was born and many early Christians didn't celebrate his birth. The Puritans banned the celebration of Christmas for a period in the 17th century, and the Catholic church was hesitant to embrace it as well. Christmas wasn't even widely celebrated in the United States until the 1950s.
None of this is to say that there's anything wrong with celebrating Christmas as a Christian holiday. Rather, I think historically it's important to frame Christmas as what it is - a sort of holiday fruitcake (holiday meatloaf?), with various chunks from different periods in human history. For that reason, it's okay for it to be different things for different people; someone saying "Happy Holidays" does nothing to dilute the tradition for the millions of Christians in the United States just as their tradition of churchgoing does not affect my secular celebration.
Christmas, for us, is a time to spend with our families, to drink (as our ancestors did!), to give and receive thoughtful gifts, and to connect with our shared human existence and history. For the entirety of recorded history, humans have been celebrating the end of the old year in preparation for the new, and it's possible to enjoy that celebration with or without the influence of Jesus and Christianity.
So Merry Christmas and Happy Holidays, friends, however you celebrate or don't celebrate.