I consider myself Christian, but I don’t celebrate a lot of major Christian holidays, and several other holidays, for reasons that aren’t really relevant to this thread. So I don’t do Christmas, Easter, Halloween, Valentines Day, etc. I’ve not celebrated any of them for at least a decade, some even longer, so everyone in my immediate family is well aware of this.
So, for people that don’t know or forget, I’m not bothered at all. After all, most people celebrate Christmas, and I can just say something like “Have a good one” and most people might even think I mean it as a response in kind even if it’s actually just a generic pleasantry. I never take offense, and I generally don’t even tell people unless it’s actually relevant for some reason. So, for instance, recently people would ask me what I was doing for Halloween, and I’d just say I don’t celebrate it. If they’re curious why, I’ll explain, if not, I’ll leave it alone.
That said, for people that DO know… frankly, it’s really irritating. I totally get people being confused and curious when it was a new thing. Or maybe thinking they’re being clever with “I know you don’t Celebrate Christmas, but have a Happy non-denominational winter celebration” or whatever. But, to me, that’s just like telling a devout Jew that they should eat pork, or Muslim that he should break his Ramadan fast to have lunch or come have drinks for happy hour, or when I’ve seen people tell atheists that they really DO believe in God they’re just angry at him. It’s disrespectful of people’s beliefs. I’m happy to answer honest questions about why I believe what I believe; after all, I wouldn’t have changed my beliefs without what I think is good reason. Worse, is that some family members I have to re-explain why I don’t celebrate it, why I won’t do it and not really mean it, yadda yadda… EVERY YEAR.
That said, I do have some friends who just want to do something nice and maybe just don’t really get it. For instance, I have a friend who just says they want to buy me a gift. If it makes them feel better, I’ll accept it. I know they mean well, and some people just are a bit oblivious to things that are outside the typical worldview.