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.