Preview at WWDC likely in June, followed by September launch.
Apple Seeds First OS X 10.11.6 El Capitan Beta to Developers
The OS X 10.11.6 update, build 15G7a, can be downloaded through the Apple Developer Center or through the Software Update mechanism in the Mac App Store.
We don't know what improvements the sixth update to OS X El Capitan might bring, but like prior updates, it's likely to focus mainly on performance improvements and bug fixes to address issues that have popped up since the release of OS X 10.11.5. According to Apple's release notes for the update, OS X 10.11.6 improves the stability, compatibility, and security of Macs.
OS X 10.11.5, the previous update, was also minor in scale and offered only bug fixes with no obvious outward-facing changes.