You can pre-order the 2018 iPhones on September 14, according to Macerkopf (a german Apple news website).
Several German mobile carriers said this to Macerkopf. With this information we can probably expect an announcement some days earlier. Possibly on September 11 or 12.
We know from the past when Apple introduces their new iPhones. That's why could state that the new iPhones will be announced on September 12 or at least not far from that. As you can see below it shows the dates of the announcements.
2010 - Monday, June 7
2011 - Tuesday, October 4
2012 - Wednesday, September 12
2013 - Tuesday, September 10
2014 - Tuesday, September 9
2015 - Wednesday, September 9
2016 - Wednesday, September 7
2017 - Tuesday, September 12
As we already know Apple will probably be unveiling three new iPhones this year. And there is a lot of excitement about this, also because the new 6.1" model will be much cheaper. This will get more people to buy one this year, it's will have almost the same specs, but it will have a LED screen and one camera.
We hope we are right and that we can expect a launch on September 12. And this isn't too long away from today! And maybe you will have your iPhone already in your hands on September 21 (probably the official release date).