The past years has been an indicative that the 2018 iPhone will be presented on September 12th and will go on sale later that month.
iPhone X, iPhone 8 and iPhine 8 Plus were presented on September 12th 2017, as per Apple’s tradition. The company, in fact, usually presents new smartphones on Tuesday or Wednesday, and almost never later than the second week of September, as highlighted by CNET.
For the last six years, Apple held its iPhone announcements on either a Tuesday or Wednesday, and usually no later than the second full week of September. For 2018, this gives us four possible dates: Tuesday, Sept. 4; Wednesday, Sept. 5; Tuesday Sept. 11; and Wednesday, Sept. 12.
Taking into account Labor Day, which falls on the first Monday of September. Looking at the last six years, according to when this day fell, Apple presented the iPhone the following week or Wednesday immediately two days after Labor Day.
This year, the most accredited date would be September 11, but for obvious reasons it is a day to be dedicated to commemorations and not to the presentation of a new product. For this reason, the most likely day for the presentation of the new iPhone might be Wednesday, September 12th. Press invitations should sent out in mid-August.
With a presentation scheduled for September 12, it is likely that the new iPhone will be launched on the following Friday, ie on September 21st.