Preview at WWDC likely in June, followed by September launch.
Apple Seeds First OS X 10.11.1 El Capitan Beta to Developers
The new beta, build 15B17c, is available through the Software Update mechanism in the Mac App Store and through the Apple Developer Center.
It is not clear what improvements the first update to OS X El Capitan will bring, but it's likely to include bug fixes and performance improvements for issues that have been encountered following the operating system's official release.
Update: OS X 10.11.1 includes support for Unicode 8 and a range of new emoji like taco, burrito, cheese wedge, hot dog, middle finger, and unicorn head. The emoji are also included in iOS 9.1, which is currently in testing as well.