According to a quote on Reddit.com, League of Legends Dominion will be released on September 8th.
Of course, this could be a rumor (just how credible is Reddit?), but if the Reddit post is reliable, then “The september 13 (november issue) pc gamer mag will feature dominion strategy (dominion having been released on september 8, with the culmination of a months-long lore cycle) and will also feature this code [Caitlyn’s Arctcic Warfare Skin].”
The free Caitlyn Arctic Warfare skin code has been confirmed, but whether or not League of Legends: Dominion will launch on September 8 is yet to be seen. Since the cover of the magazine (which releases September 13) features a Dominion strategies article, I can only assume that it’s a fairly accurate assumption that Dominion will release before that time.