Preview at WWDC likely in June, followed by September launch.
Apple Releases Second OS X Yosemite Golden Master Candidate to Developers, Public Beta 5
The latest Yosemite update, build 14A386a, can be downloaded from the Mac App Store and through Apple's Mac Dev Center.
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, which is expected in late October.
The current build, like the previous golden master candidate, may not be the final version of OS X Yosemite, depending on the outcome of final testing. Last year, Apple seeded multiple golden master versions of OS X Mavericks, with the final version coming a couple of days ahead of the software's public release.