The seventh beta of iOS 13 was released today. Inside was a screenshot named HoldForRelease, with the calendar icon set to Tuesday the 10th.
That’s a pretty good indicator that the Apple Event and release of the new iOS will take place on September 10, 2019. This was spotted by site iHelpBR, who figured out the September 12th event last year. It’s possible Apple would do this to throw people off, but it seems unlikely – the date makes a lot of sense for the event to be held.