"Christ" is literally in the name. They can be secular, but it is taking something that was a religious symbol for a very long time and making it secular.

My name is from the Bible. Most people don't realize that. Plenty of atheists have that name including at least one very prominent one. That doesn't make me dislike that fact any less and it absolutely informed our decision not to give our daughter a religious name.

Also, while it does happen, very few people who do not come from families that were at least Christian in the relatively recent past generally don't have Christmas trees. At least not in the U.S.

I've known Jews and Muslims who do it, but they are rare and often, like me, married to someone with a Christian background. At least here in the U.S.

I'm not criticizing people who celebrate it, but I can't control what makes me uncomfortable. Coming to something common as a complete outsider for the first 20 years of your life can make things very uncomfortable. Imagine if everyone around you celebrated Ramadan. You might eat during the day, but you'll probably do so being totally aware that you're an "other" if you do it. But if you fast like everyone else after not doing it your entire childhood, you might find it very uncomfortable. (Psychologically, I mean. The physical discomfort of fasting is a separate matter.)

We didn't do Christmas. No Christmas trees, no wreaths, no Santa, no carols, nothing. And, of course, that meant that other kids bullied me because of it. Which only made me feel even more like an outsider. I'm not going to say why, it involves severe bullying, violence and antisemitism, but the one Christmas thing I cannot tolerate in my house is the song Rudolph the Red Nosed Reindeer. It literally hurts to listen to. When kids bully you and include a Christmas carol in the bullying, that alone can make Christmas uncomfortable.

Like I said, no criticism for people who celebrate it. I even celebrate it now because of my wife and daughter. But I don't enjoy it. It's a sense of duty. If I were single, I would ignore both Christmas and Easter.

They are though. Our culture is Christian. It’s an aspect of cultural Christianity

There’s a term: Christian atheist. It refers to people who are atheists but still partake in a lot of the Christian cultural practices and values. There’s nothing wrong with that, but be aware that that will cause differences with Jewish atheists, Muslim atheists, and atheists who are dechristianizing themselves.

When you say that atheists can still celebrate Christmas be aware you sound like someone saying atheists can and should celebrate Ramadan. Neither of you is wrong, but you are bringing a religion and culture into it.