The simple solution is to add a new user to the "server" Mac, then load all the holiday music into iTunes running under that user. Share the music using a distinct name, like "Christmas Music." You can share the music from multiple copies of iTunes on the same Mac under different users, so it won't interfere with the usual music library sharing.
At the end of the Christmas season, back up the music (or the entire user), delete it, and then put it away until next year!
[robg adds: I haven't tested this one (with "this" being multiple users sharing music from one Mac at the same time). Here I just keep the holiday music unchecked in the main library (and my other smart playlists are set to play checked songs only), and use a smart playlist set to the Holiday genre for listening to holiday music.]

