The usual pattern that Apple has followed as of late is to unveil the new iPhone on the first or second Tuesday or Wednesday of September, and then ship the iPhone second Friday following the event, or, as was the case with the iPhone 6s in 2015, the third Friday.
However, the current rumormill is suggesting that while the iPhone 8 will go on pre-order in September, it won’t be ship until October or perhaps even November.
This would allow Apple to not only get the regular and Plus iPhones out of the door, but also gauge demand for the iPhone 8, as well as catch up on shortages due to low component yields.
There are some going as far as to suggest that the tenth-anniversary iPhone 8 won’t ship this year. This seems unlikely to me.
Another possibility is that that the iPhone 8 will launch with some features disabled because the software for some features won’t be ready. As bad as this sounds, it’s better than having buggy features.