Preview at WWDC likely in June, followed by September launch.
Apple Seeds Third OS X 10.11.5 El Capitan Beta to Developers and Public Beta Testers
OS X 10.11.5 beta 3 has a build number of 15F28b and can be downloaded through the software update mechanism in the Mac App Store or through the Apple Developer Center (for developers).
OS X 10.11.5, like prior OS X El Capitan updates, appears to focus on security enhancements, performance improvements, and bug fixes to address issues that have popped up since the release of OS X 10.11.4. No obvious outward-facing changes were discovered in the first two OS X 10.11.5 betas, but we'll update this post if any new features are found in the third beta.