Apple has not yet announced when iOS 12 will be released, but release dates in the past years provide some clues as to when iOS 12 will be released.
With the WWDC 2018 event held in June last year, Apple announced that the mobile operating system version of iOS 12 introduced. iOS 12, which brings features such as Memoji, Screen Saver, Siri Shortcuts to iPhone, goes through its last days in beta
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Apple did not officially explain when it would publish a stable version of iOS 12 to users. But the information you have provided gives us enough clues to figure out what day Apple is going to upgrade to iOS.
iPhone 7 and iPhone 7 Plus were introduced on September 7th and iOS 10 was released on Tuesday, September 13th.
iPhone X, iPhone 8 and 8 Plus were introduced on September 12 and iOS 12 was released on Tuesday, September 19.
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Apple will launch the 2018 model iPhones on September 12th. If we look at the references we got from previous years, we can say that the company's mobile operating system will release the new version iOS 12 on Tuesday, September 18.