Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Either Sept 6th, or 13th, which is the second Tuesday like usual. Apple nails down their figures for sales, App Store downloads, etc., on Mondays and inserts the latest data into the presentation for Tuesday.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.