There are many Atheists who celebrate Christmas and many who don't. Many feel it has outgrown any religious context, others think the religious aspects are stronger than ever. Some hear the pleasant sounds of the season creating a wonderful atmosphere, others can't stand the noise and constant praising of a baby savior.
Regardless how you look at it, Atheists think about Christmas very differently than Christians do. As there are many people out there who are going to be celebrating Christmas with an Atheist, I felt it only right to answer a very important question. Keep in mind, I will be speaking from personal experience as I cannot speak for everyone around the world.
"How do Atheists celebrate Christmas?"
Not surprisingly, it isn't very different from how a lot of people celebrate Christmas. We enjoy the music, love to give presents, and are overall happier during this time of year, if celebrated correctly. However, there are some major differences that may be present, and they change from person to person.
1. The music
There are three different categories that you must be aware of. There are those of us who hate the music this time of year specifically because it is played on repeat for three months straight. There are also those who love all of the music and appreciate the culture that surrounds this holiday. The most prominent, however, are those that enjoy the "X-Mas" songs but not the "Christmas" songs.
Essentially, if they hear anything about Jesus during this time of year they will want to change the station or skip the track. Typically they will enjoy the music, just not the lyrics. Some can ignore the lyrics and just concentrate on the music, but most of them refuse to listen to them at all. As I said, though, not all songs are bad. Jingle Bells, Frosty the Snowman, Up on the House Top, these songs are all great and remind them of the joyous times ahead.
Good rule of thumb, if you'd typically find it sung by a choir, try to get an instrumental version of it. At least then you have some leeway and can learn what kind of Atheist you are celebrating with.
2. The decorations
At a church you will typically see a manger scene, perhaps an angel on top of a tree, various obviously religious aspects. These are things you will rarely find in an Atheist household, for obvious reasons. Again, there are those who don't mind these decorations and actually find them to be rather lovely in the right placement, then there are those who despise anything that even remotely relates to the religious side of the holiday. And, just like with the music, you'll mostly find those that are half-way between these states.
If your Atheist enthusiastically puts up a Christmas tree, they are probably not 100% against these decorations. Watch what they start putting on the tree or what they buy for decorating their house. These are important clues and can even help in determining the other aspects they like about the holiday. If they buy a star and not an angel, try to avoid obviously religious aspects. If they do buy angels, but not too many, don't be afraid to talk about the religious aspects or get them something that looks nice just because it is religious, but don't over-do it.
If you'll find it at a church, steer clear until you know they won't mind.
This is rather easy to guess most of the time. The majority of Atheists won't go to any church or participate in any religious activities. However, there are a few that will for various reasons. Some attend just to make fun of the religious (best to avoid the situation altogether) and some are more than willing to participate if they have family or friends that enjoy them. And then there are some that refuse outright.
There is no majority in this case, so you'll have to play this one by ear. Watch what they do and make casual mentions here and there and you'll usually get an honest answer out of them before long without any feelings getting hurt.
4. The rest
You can pretty much guess all of the minor details from here. A lot of the time you won't even know an Atheist who celebrates Christmas from anyone else who does. They enjoy a lot of the same activities, music, and decorations, with only minor changes. Once you can pick out these minor changes, you'll know the kind of Atheist you are celebrating with, and everyone involved can have a much more enjoyable holiday season, even if it is a little more secular than you're used to.
In the spirit of giving, this year I've decided to throw in a bonus question.
"Do Atheists celebrate Kwanza and other religious holidays?"
Just like with Christmas, a lot of Atheists will celebrate these other religious holidays, around the world and throughout the year, if they have grown up with them and can still appreciate the greater meaning behind them. They will celebrate them differently most of the time, with the differences being similar to what I described above, but they will usually see nothing wrong with maintaining a tradition they grew up with, as long as it can be modified to suit their new beliefs, or lack there of.
Happy Holidays, everyone!