Preview at WWDC likely in June, followed by September launch.
Apple Seeds Second Beta of macOS High Sierra to Developers
The new beta can be downloaded from the Apple Developer Center, and once installed, subsequent betas will be available through the Software Update mechanism in the Mac App Store.
macOS High Sierra introduces new core storage, video, and graphics technology. The update supports the new Apple File System (APFS) and High Efficiency Video Codec (HEVC), plus it introduces an updated version of Metal with support for VR and external GPUs.
Photos is being updated with a new sidebar that makes it easier to access editing tools and albums, and there are new editing options like Curves and Selective Color. It also supports external editing apps like Photoshop and Pixelmator, saving changes made in those apps.
Improvements to Safari are coming, including speed enhancements and a new feature that puts a stop to data tracking. Siri in macOS High Sierra has expanded music capabilities and a new, more natural voice, and Spotlight supports flight status information. There are also improvements to iCloud, FaceTime, Messages, and Notes.
macOS High Sierra is available only for registered developers, but Apple plans to make a public macOS High Sierra beta available later this summer, giving public beta testers a chance to try out the software before it sees a public release in the fall.