Publish a book in December if you can help it, unless the story is holiday-related. Seriously. It will get lost. Readers won't associate it with the year in which it was actually published (because, in their minds, they've done all their "serious" reading for the year and are focused on holiday offerings), but they won't associate it with the following year, either. I've seen how Xylophone slipped through the cracks because it was published on December 12.
You're better off waiting until January.