Frelorn provided us with an update regarding the start of the September event:

Update: We are changing the day of the September patch from Tuesday the 18th to Thursday the 20th. We ran into some issues that we felt needed to be addressed before we would ok the September patch.

This means that the release notes as well as the fiction, will now come out on Tuesday the 18th.

For more info on AC, check out AC WarCry.