iOS 9 will be released on September 16th.

Apple's new mobile operating system finally has a release date. iOS 9 will be available on September 16th across the world. The date was announced on stage at the company's latest product keynote, alongside new hardware including the Apple TV and the iPhone 6S and 6S Plus. iOS 9 was first announced at WWDC in June, and was made available as a public beta the following month, but this will be the first official release for the operating system.
The company did not specify a time of day for the release, but Apple traditionally releases upgrades at 1PM ET and is expected to do the same with iOS 9.
Most of the upgrades in the new iOS focus on making iPhones smarter, faster, and more efficient. The biggest change is the iPad's new multitasking mode, which allows users to run two apps side by side simultaneously, similar to features in Surface and Galaxy tablets. Siri is also getting an upgrade, as a new feature called Proactive will offer contextual user suggestions based on habits, location, or time of day. Spotlight searches will also get more powerful, allowing users to search content within apps and perform unit conversions without opening a web search.
For anyone upgrading immediately, the biggest difference will be size and speed. The iOS 9 package requires only 1.8GB of free space to install — less than half of what iOS 8 required — and the new low power mode will let users squeeze an extra three hours of battery life out of a single charge.