This morning, BGR is reporting that Apple is ready to release iOS 7 beta 6 to developers next week. The exact day is unknown, but most beta build are being released on Mondays.
After iOS 7 beta 6, Apple will be seeding a GM (gold master) version for its employees and partners to test starting around September 5th. This will mostly likely be the software that is released to the public later on in the month of September, barring any major bugs or problems that might be discovered.
After Apple and its partners are comfortable with the gold master build, the company will release it to developers on September 10th, the day of the company’s iPhone event where we should see the new iPhone 5S and iPhone 5C make their first appearances — and they will of course be the first devices to run iOS 7 out of the box.