The point is that
everyone has feelings. Christians are saying to non-Christians, "Of course you are welcome. You have your own religious functions, and your own events with your personal friends and relatives..." And non-Christians are saying the same thing back to Christians.
While December is a great example, I was thinking more along the lines of what we see generally happening in society when holidays aren't even the issue. A minority say, "My feelings are hurt" and today it is the majority who are expected to back-off. It does not matter that the feelings of the majority are now the feelings that are hurting because what is important and momentous to them is being downplayed by the minority. The majority are dismissed with a "You shouldn't feel that way" when no one dares offers that "advice" to the minority view.
Ironically, what each side is saying to the other is exactly the same:
"You shouldn't feel that way."
As to your specific example, the Christian should have accepted that whoever planned the party chose to say "Holiday Party" (unless of course that was mandated, but let's say it was not). She should have gone to the holiday party, had a good time, and then volunteered to head the party next year when she could choose what to call the event.
Or, better yet, perhaps the issue could be resolved by just saying "Party."