Judging from David Smith on Under the Radar Podcast - last year, sometime around 10PM on GM release day, iTunesConnect started taking submissions.
Check out his latest podcast for dealing with launch day stuff.
Gold Master was just released - grab it, build with it, and test. Only builds built with this GM will be accepted.
This came in over the RSS feed at noon:
You can now submit your apps that take advantage of exciting new features available in the next release of macOS, iOS, watchOS, and tvOS. And with the new App Store for iMessage, users will be able to download your iMessage apps and sticker packs directly within Messages.
Build your apps using Xcode 8 GM seed, test with the GM seeds of macOS Sierra, iOS 10, watchOS 3, tvOS 10, and submit them for review. To learn more about preparing your submissions for the App Store, see Submitting Your Apps.