Preview at WWDC likely in June, followed by September launch.
Apple Releases Third OS X El Capitan Public Beta
The third public beta follows the launch of the fifth OS X El Capitan developer beta, which was seeded to developers late on Monday afternoon. No outward-facing changes were introduced with the beta, suggesting it's an under-the-hood update with bug fixes and performance improvements.
Beta testers who have signed up for Apple’s beta testing program and already have the public beta installed can update through the Software Update mechanism in the Mac App Store for OS X.
OS X 10.11 El Capitan is expected to be released to the public in the fall after the beta testing process is complete.