iPhone 8 could be released in September, but you may have a hard time finding one
According to MacRumors, which cites information revealed by several Barclays analysts, the iPhone 8 will be both announced, and released in September - alongside regular iPhone 7s and iPhone 7s Plus models (these should be direct successors to the iPhone 7 and iPhone 7 Plus). However, Barclays analysts have noted that the fancy iPhone 8 would likely not be widely available until the fourth quarter of the year, thus starting October. In other words, while the handset should be launched at the same time with the (presumably less exciting) iPhone 7s series, it will probably be sold out in no time.
The iPhone 8 is expected to have a 5.8-inch display, but, thanks to the almost non-existent bezels, it could be about as small as the current iPhone 7 model (which sports a 4.7-inch screen). You can take a look at what the iPhone 8 might look like here and here.
As a reminder, iPhone 8 might not be the final name of this device. Apple may want to go with iPhone Pro, iPhone X, or something else. We'll just have to wait and see what happens.