They will announce at the September event, or in October, along with new case materials, and then they will go on sale in time for Christmas, probably Black Friday in November. That gives them enough lead time to ensure enough stock for stores -- I doubt they will want a repeat of the launch. Nor do they want to give their competitors too much time to beat them to market.
Among the new watch cases will be anodized gold for the Sport to match a customers gold iPhone, as well as gold clad watches in the $1K-2K range. This gives them enough time to sell out the limited run of Edition gold watches, before the general public has one that looks similar, at the same time offering a new limited run of platinum Editions.
I would also expect they would announce the ability to pair more than one watch with the same phone, thus encouraging people to buy multiple watches to switch out day to day to match outfits and events. Hubby already bought an Watch, and don't know what to get him for Christmas? No problem, get him an affordable gold-clad for dressier occasions, to match his grandfather's gold cufflinks.