I think it's going to be the Sept. 5/6. Looking at the previous years, there is a noticeable trend that seems to be taking place. Apple shifts things up every two years.
In 2012-2013, that both invites were sent right after Labor Day (Sept 3, Sept 4, respectively - both of which are Tuesdays). The announcement date was about a week later.
In 2014-2015, Apple changed this to right before the Labor Day weekend (Aug 28, Aug 27, respectively - both of which are Thursdays). The announcement date was a little less than two weeks later.
In 2016, Apple once again changed this to exactly one-week before labor day (Aug 29, which is a Monday), with an announcement date set for the following week.
Notice that trend there? It seems like Apple changes things up every TWO years. Of course it could all be random or Apple might change it up this year because it's the 10th anniversary, but speaking strictly based of that trend - we should expect an invite on Monday, for an event the following week (ie. Monday, Aug 28 = invite date; Sept 5/6 = announcement date).
Further, we also notice that the dates keep going back. 12 September 2012 > 10 September 2013 > 9 September 2014 > 9 September 2015 > 7 September 2016.
I'm putting my money on Sept. 6. Further the MWC date has been known for a few months now, so Apple must have been aware of it. Unless they don't give a **** ...