Apple released the seventh beta of iOS 13 today, and that release contained image files that seem to indicate that Apple will hold its next big iPhone unveiling event on September 10, 2019.
A screenshot of iOS in the beta labeled "HoldForRelease" was originally found by iHelpBR but shared on MacRumors and elsewhere. The image includes an iOS Calendar app icon that says September 10 on it. A similar screenshot was found saying September 12 shortly before Apple's iPhone unveiling event last year. That event was held on—you guessed it—September 12. Also this time around, versions of the image with the date on them were found depicting both the iPhone and iPad interfaces.
Of course, this date was likely anyway. The current incarnation of Apple almost always holds its iPhone event around the same time each September. The event is commonly in the second week of September and usually on a Tuesday. Last year's event was on a Wednesday presumably because the Tuesday fell on September 11. (It was Tuesday, September 12 in 2017.)