Preview at WWDC likely in June, followed by September launch.
Apple Seeds Fourth OS X El Capitan Beta to Public Beta Testers
The fourth public beta follows the launch of the sixth OS X El Capitan developer beta, which was seeded to developers yesterday. Aside from new wallpaper, no outward-facing changes were introduced with beta six, suggesting it's largely 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.
OS X 10.11 El Capitan is expected to be released to the public in the fall after the beta testing process is complete.