Preview at WWDC likely in June, followed by September launch.
Apple Releases Second OS X Yosemite Public Beta, iTunes 12 Update
The software update, build 14A329r, can be downloaded through the software update function of the Mac App Store.
It's likely the new public beta bundles the content introduced in the last several developer updates, including multiple design changes added with DP 6. The last beta included new wallpapers, a new System Preferences design, a translucent dashboard, several icon updates, and more.
Apple is expected to release OS X Yosemite to the general public later this year, possibly following an October event that may also introduce both the iWatch and new iPads.