Preview at WWDC likely in June, followed by September launch.
Apple Seeds Second OS X 10.10.5 Yosemite Beta to Developers
Today's beta, build number 14F19a, can be downloaded through the Software Update mechanism in the Mac App Store or through the Apple Developer Center.
The first OS X 10.10.5 beta did not appear to include any consumer-facing changes, so it's likely that this update focuses on behind-the-scenes bug fixes and performance enhancements.
Over the course of its lifespan, OS X Yosemite has received four updates. The first two brought bug fixes and security improvements, while OS X 10.10.3, the most significant update to Yosemite, introduced the new Photos app, Force Touch APIs, new emoji features, and more.
OS X 10.10.4, the current version of Yosemite, also focused on bug fixes and stability improvements, most notably introducing TRIM support for third-party SSDs and eliminating the Discoveryd process that caused ongoing connectivity issues in the OS.