Preview at WWDC likely in June, followed by September launch.
Apple Seeds Third OS X El Capitan 10.11.1 Beta to Developers and Public Beta Testers
The new beta, build 15B30a, is available through the Software Update mechanism in the Mac App Store and through the Apple Developer Center.
The first beta of OS X 10.11.1 introduced support for a range of new emoji included in Unicode 8, like cheese wedge, taco, burrito, popcorn, middle finger, unicorn head, and more. Other betas appear to include under-the-hood fixes that are not immediately apparent.
It is not clear what the first update to OS X El Capitan will bring to the new OS aside from updated emoji, but it's likely to be a minor update that includes bug fixes and performance improvements for issues encountered following the operating system's official release. According to Apple's release notes, the beta offers stability, compatibility, and security improvements.