Media event set for September 9, launch likely ~10 days later.
Apple Asks Developers to Update Apps for Mavericks Ahead of October 22 Media Event
Make sure your app takes advantage of the great new features in OS X Mavericks when the world’s most advanced desktop operating system becomes available to millions of customers later this fall. Download OS X Mavericks GM seed and Xcode 5.0.1 GM seed, now available on the Mac Dev Center. Build your apps with these latest seeds, then test and submit them to the Mac App Store.At the beginning of October, Apple began training its AppleCare staff on OS X Mavericks and the company has publicly stated the OS will be coming later "this fall".
The release of the new OS is expected to be announced during the media event on October 22.