Preview at WWDC likely in June, followed by September launch.
Apple Seeds Second OS X 10.11.6 El Capitan Beta to Public Beta Testers
Beta 2 is available through the Software Update mechanism in the Mac App Store for those who are enrolled in Apple's beta testing program. Mac users who want to join the program can sign up on Apple's beta testing website.
OS X 10.11.6, like most of the previous updates to the El Capitan operating system, appears to be minor in scale, focusing mainly on bug fixes and security improvements that are not immediately obvious to testers and developers. No outward-facing changes or major bug fixes were discovered in the first two developer betas.