Preview at WWDC likely in June, followed by September launch.
Apple Releases Third OS X Yosemite Public Beta, Developer Preview 8
The software update can be downloaded through the software update function of the Mac App Store, and developers can also download DP 8 from the Mac Developer Center.
It is possible given the release timing that both versions of Yosemite, DP 8 and the new public beta, are the same. The public beta will also include updates from Developer Preview 7, which included a new look for Dark Mode, several tweaked icons, and the removal of the "Software Update" option from the main Apple menu.
Apple is expected to release OS X Yosemite to the general public later this year, possibly following an October event that may introduce new iPads.