https://www.theverge.com/2017/9/12/16278316/apple-watch-update-watchos-4-features-release-date

Apple today revealed the release date of watchOS 4, the next iteration of its Apple Watch mobile operating system, during its annual fall hardware event. The software, first announced back in June during WWDC, will come out on September 19th, coinciding with the third-generation LTE-equipped Apple Watch Series 3. The software, while not as big of a change as the jump to watchOS 3 last year, will come with new Siri features, a set of new watch faces, and some improved fitness tracking capabilities.

The company also took time to detail some new features that weren’t talked about back during WWDC, including a slew of new heart rate monitoring features. The company’s smartwatch, which Apple COO Jeff Williams says is the most used heart rate monitor in the world, will now show your heart rate on the Apple Watch’s face, so you can view it at a glance when you raise your wrist.