Historical iOS GM release dates:
|iOS Version||Release Date||Day of the week|
|iPhone OS 1||29th June 2007||Friday|
|iPhone OS 2||11th July 2008||Friday|
|iPhone OS 3||17th June 2009||Wednesday|
|iOS 4||21th June 2010||Monday|
|iOS 5||12th October 2011||Wednesday|
|iOS 6||19th September 2012||Wednesday|
|iOS 7||18th September 2013||Wednesday|
|iOS 8||17th September 2014||Wednesday|
|iOS 9||16th September 2015||Wednesday|
|iOS 10||13th September 2016||Tuesday|
From iOS 5 onwards there is an obvious trend - midweek release in the third week of September. Based on that, and the move to Tuesday last year, I will go with Tuesday, September 12th as the release date for iOS 11.
Update - I was a week off. iOS 11 releases to the public on the 19th September.