It's completely specialty and program dependent. Some specialties will start interviewing in the middle of October, others won't even get started until the end of November or early December. Even within specialties there is a lot of variance.
Additionally this year is liable to be different from previous years since this is the first time that the application is submitted on 9/14 instead of 9/1 and that the MSPE goes out on 10/1 instead of 11/1. Because of this, several people have suggested that it'll be pretty quiet until 10/1 since programs don't lose much by waiting a couple of weeks to look at the MSPE; I'm inclined to agree with that, but it's really anyone's guess.