Preview at WWDC likely in June, followed by September launch.
Apple Seeds First OS X Mavericks 10.9.5 Beta to Developers
OS X 10.9.5 follows new seeds of Safari 7.0.6 and 6.1.6 for Mavericks and Mountain Lion, which were released to developers last night.
It is unclear what improvements the 10.9.5 update will bring to Mavericks, but it is likely to include bug fixes and stability enhancements. Apple is asking developers to focus on USB, USB Smart Cards, Graphics, and Safari.
Along with working on improvements to Mavericks, Apple is also beta testing OS X Yosemite, which is due to be released in the fall. The last Mavericks update, 10.9.4, added several Wi-Fi fixes and improved wake from sleep reliability.