Preview at WWDC likely in June, followed by September launch.
Apple Seeds Third WatchOS 2 Beta to Developers
watchOS 2 requires iOS 9 and can be downloaded over-the-air through the Apple Watch app on the iPhone by going to General -> Software Update.
watchOS 2 is the first major update to the software that runs on the Apple Watch. The new software brings many new features that Apple Watch developers can take advantage of, including native apps that run on the Apple Watch itself instead of the iPhone. It also allows developers to access the Taptic Engine and a number of other sensors in the Apple Watch, including the heart rate monitor, the microphone, and the accelerometer.
Along with these features for developers, which will lead to much more complex and full-featured third-party Apple Watch apps, the update introduces new watch faces (photo albums and time-lapse), third-party Complications, Time Travel for viewing upcoming events in a watch face Complication, and a Nightstand mode that's activated when the watch is charging, displaying the time.
watchOS 2 is currently only available to registered developers who have also installed iOS 9 on their iPhones. Apple plans to release the software to the public in the fall.