Preview at WWDC likely in June, followed by September launch.
Apple Seeds First macOS Sierra 10.12.1 Beta to Developers
macOS Sierra Beta 10.12.1 is available for download through the Apple Developer Center and through the Software update mechanism in the Mac App Store.
It is not clear what improvements the first update to macOS Sierra will bring, but it's likely to include bug fixes and performance improvements for issues that have been encountered between the time between the beta testing period and release.
We'll update this post with any new features that are found in macOS Sierra 10.12.1.