I doubt they'll do anything for Christmas.
They never actually do anything on Christmas, since I suppose everyone's on holiday leave. In previous years when they had an active game out that they owned, they'd support it with something fun either on the site or in-game (for a small example, Valhalla usually had some sort of graphic on the metallic wall), but I can't imagine much of anything this year.
Bungie said they'd be going dark for longer than they ever have before, and certainly for longer than anyone who's entered the community since Halo 1's release will be used to. I don't think we'll see anything from them for a while. But I do figure that if and when we do hear from them, they'll make it big and worthwhile.
So, I'm fine with them not doing anything for the holidays.
Also, for those above saying not everyone celebrates Christmas; true, but it's still a national holiday in many countries, including Bungie's own, so it's not exactly irrelevant.