According to CNET, based on Apple’s past iPhone announcements, this has typically taken place on a Tuesday or Wednesday and no later than the second week of September. This means that in 2018, this gives us four possible dates to consider which are the 4th and 5th of September, or the following week on the 11th or 12th of September.
However CNET notes that Labor Day falls on the first Monday of September (for the US) and based on previous Labor Days, Apple usually chooses to launch their iPhones the following week, which is what they did back in 2015 and 2016. This means that this leaves either the 11th or 12th as the most likely candidates (dates, get it?).
Of course this is pure speculation at this point, although given Apple’s past launches, it’s hard to argue against the logic. Either way we’ll have to wait and see but if Apple is indeed planning an early September announcement, we suppose invites will start going out either towards the end of August or possibly the first week of September.