Preview at WWDC likely in June, followed by September launch.
Apple Releases Sixth OS X Yosemite Public Beta
The software update can be downloaded through the software update function of the Mac App Store.
OS X Yosemite brings a flatter, more modern look to OS X, with an emphasis on translucency and redesigned dock, windows, and more. It also includes a multitude of new features, such as improved integration with iOS 8 through Continuity, a new "Today" view in Notification Center that offers integration with third-party apps, a retooled Spotlight search with new data sources, and several new features for apps like Mail, Safari, and Messages.
Over the course of the beta testing period, each Developer Preview has added new features and refined the look and performance of OS X Yosemite, although changes have become less obvious in more recent builds as Apple begins finalizing Yosemite ahead of its public release.
Apple is expected to launch Yosemite soon after the iPad event that it is planning to hold this Thursday.