At this point, seemingly everything there is to know about the iPhone 8 has been leaked, so it seems perfectly appropriate that the release date for the next device that you'll be texting with, taking pictures with, and (on rare occasions) holding up to your ear, would be leaked as well.
According to French website Mac4Ever, the launch event for the phone will take place on Tuesday, September 12, with pre-orders becoming available on Friday, September 15 and the full release happening a week later. The site further went on to claim that phone operators have been given advance notice of these dates so that they can handle the predictable traffic that will coincide with the phone's release. In previous releases, Apple has typically put the phone up for sale the week after they started taking pre-orders (which they also typically do on Fridays).
At the aforementioned launch event, Apple is expected to unveil three phones, including the marginally improved iPhone 7s and 7s Plus, in addition to the iPhone 8.
Feel free to catch up with all the rumors surrounding the latter device right here.
Also, as the Independent notes, Mac4Ever has had pretty solid success with Apple leaks in the past, though it's more than worth noting that the release date and phone operators couldn't be confirmed. Additionally it's worth noting that some rumors have stated the iPhone 8's will be delayed, due to problems they're having putting them together. Lower your guard accordingly.