It hasn't been officially announced yet, but it's most likely September 21 or 28. Sept. 28 is the first date an episode of Simpsons is currently planned, but that would only allow two episodes before the world series, so I'm thinking the 21st is more likely...
EDIT: Doing a little more research, it seems that the 28th is the season premiere, and there indeed is an episode on October 12th, as I guess the series begins on the 13th. (thought it was a week prior) The Treehouse of Horror episode, naturally, will air on November 2nd...
