Easier to just say "merry christmas" back, if that's their particular poison, and they get the first word in---hell, why not?  Same if they say happy kwanza, hannukah, festivus of whatever...

Here's to the birth of the New Sun, the return of light, the season of giving and trying to be nice to each other---Let's lower our swords for a day and see what that's like.  The results may surprise us.