Imitating disbelievers (tashabbuh) is forbidden:
...the Messenger (blessings and peace of Allah be upon him) definitively prohibited it and said: “Whoever imitates a people is one of them.” -- Islam Q&A
Thus, if we imitate a disbeliever, we are essentially a disbeliever. It's quite important, therefore, to determine when one is imitating a disbeliever and when one just happens to have something in common with a disbeliever.
Question: How to distinguish "imitation" from "doing the same thing"?
Presumably, something like reading a newspaper would not amount to imitating a disbeliever, despite the fact that disbelievers read newspapers.
Something like eating hot cross buns might be more borderline, as while the cross represents a crucifix, in my experience, they're ordinarily going to be eaten without much thought of Christianity. (They're eaten here in China all year round, and I doubt they're cooked with any realization of the "cross = crucifix" symbolism.)
Also coming into play are:
Simply being friendly. It could be interpreted as being rude and elitist to have a deliberate aversion to what one might perceive as imitation.
Whether or not one intends imitation. E.g. saying "merry Christmas" out of politeness is something many non-Christians do.