It looks like Apple will release the iPhone 7 on September 23
You might be able to buy the iPhone 7 on September 23, if this leaked photo spotted by 9to5Mac supposedly showing this "reset hours" September calendar for AT&T stores is to be believed.
9to5Mac suggests that the "reset hours" could indicate times when AT&T begins advertising for iPhone 7 pre-orders on September 9, which lines up nicely with the rumors that Apple's fall iPhone event is going to take place on September 7. Apple tends to release its new iPhones roughly two weeks after it unveils them.
Since the September 23 date is the only scheduled "reset" after September 9, it looks like AT&T will sell the iPhone 7 in retail stores starting September 23.
If this leaked schedule from AT&T is in fact legitimate, it's likely that other carriers will also have the same dates lined up for the iPhone 7's pre-orders and release, but we have yet to see any "leaked" schedules from Verizon, T-Mobile, or Sprint.