You need to learn to ignore them.
They are not REQUIRED to look what you do, or to approve of it, etc. And while politeness might dictate that they not complain at you, why wouldn't they? You keep trying to please them, so as long as they make their DISpleasure known, they might get what they want.
So stop trying to get them to agree. They probably never will, and they have no motivation to, and they aren't REQUIRED to.
BUT: just because they don't like what you do, you don't have to CHANGE. You are not REQUIRED to please them, either.
So just don't indicate that you give a hoot about whether they're happy. Be blithely pleasant about your plans, and NEVER NEVER justify or explain. Just shrug and say, "Oh, well, we've made our plans."
And I'd also recomment you NOT try to jolly them along by saying, "I'm sure you'll have a nice day anyway." Don't discuss that. OR, say in an interested way, "What are YOU going to do for Christmas this year?" As if there IS an interesting answer, she's the one who is responsible for coming up with that answer, and you are eager to hear about her positive plans. If she says something about not having them, or you not being there, then say, disappointedly, "Oh, you should come up with something fun to do. I'd love to hear about your plans."
And, also, I might suggest that you just not discuss this with them. You've told them. Don't *you* bring up Christmas. If they do, just say in a slightly surprised tone of voice, "I know we mentioned it--we're going on a vacation."
And try not to think abot it between now and then? You'll be annoyed when it happens--why be annoyed so many months ahead of time?