Preview at WWDC likely in June, followed by September launch.
Apple Releases Third macOS Sierra 10.12.1 Beta for Public Beta Testers
Beta testers who have signed up for Apple's beta testing program will receive the 10.12.1 macOS Sierra beta through the Software Update mechanism in the Mac App Store.
Those who want to be a part of Apple's beta testing program can sign up to participate through the beta testing website, which gives users access to both iOS and macOS Sierra betas. Betas should not be installed on a primary machine due to the potential for instability.
macOS Sierra 10.12.1 appears to focus mainly on bug fixes and performance improvements to address issues that have surfaced up since macOS Sierra was first released. It also includes album support in Photos for the new Portrait feature in iOS 10 on the iPhone 7 Plus.
macOS Sierra is a major operating system update that introduces features like Siri support, cross device copy paste, improved iCloud functionality, the ability to unlock a Mac with the Apple Watch, an Optimize Storage option, and more.