Preview at WWDC likely in June, followed by September launch.
Apple Releases OS X 10.9.5 With Reliability Improvements, Safari 7.0.6
The update, which is recommended for all Mavericks users, improves the reliability of VPN connections that use USB smart cards for authentication and it improves the reliability of accessing files located on an SMB server. Also included with OS X 10.9.5 is Safari 7.0.6.
OS X 10.9.5 will likely be the final update to Mavericks ahead of the release of OS X Yosemite, which is expected in October. OS X Yosemite brings several improvements to the operating system, including a revamped look and several new features that improve integration with iOS.
Update: Apple has also released Safari 7.1 for 10.9.5 users. Safari 7.1 adds DuckDuckGo as a built-in option in the search field, encrypts Yahoo searches from the search field, and improves website compatibility with both Reader and Autofill. DuckDuckGo is a search engine that does not track users.