Here's an excerpt from Bungie.net:
Our multiplayer design teams are hard at work on a couple of things. One of them is a fun, seasonal addition to the matchmaking playlist which should keep you entertained for about 27 hours around Halloween. No specifics yet, but suffice it to say that if you want to spend your witchy holiday playing Halo 3, we’ll give you some spooktacular reasons to do so.
