Preview at WWDC likely in June, followed by September launch.
Apple Releases Second Beta of tvOS 9.1 to Developers
The second tvOS beta is available as an over-the-air update to those who have installed the first beta. Installing an initial tvOS beta is a more involved process than installing a beta update for iOS or OS X. The update requires the Apple TV to be connected to a computer with a USB-C to USB-A cable, with the software downloaded and installed via iTunes.
We don't yet know exactly what is included in the new version of tvOS, as most changes thus far appear to be under-the-hood updates. Apple's release notes for the update include references to networking and scrolling tweaks, and it's likely tvOS 9.1 will include fixes for bugs and issues that have been discovered in the software since the Apple TV's official launch.
We will include any other external facing changes below.