I was going to go for the September 11th date since Nintendo seem to like to release games then, but someone beat me to it. Bummer.
I can see arguments for it being revealed in September, October, and November, so it's a tough call which to choose. Since no one's really been saying November yet, I'll go with that month as it's a common time for launches of big games and systems, so why not big announcements? I could see it happening on November 18th since they could also celebrate the anniversaries of GameCube and Wii on that day, but to cover the full week my official guess will be:
November 20th, 2016