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Apple Inc. (NASDAQ: AAPL) has sent out invitations to a special event on September 12. Some press outlets have speculated the company will release a new iPhone. It is a good bet. Of the past nine major iPhone releases, seven have been in September.

From the iPhone 5 through the iPhone 8, only the SE version and the iPhone X missed the September date. In the case of the iPhone X, parts and assembly delays may have been the reason. Under the ideal circumstance, the latest iPhone would have been announced in September and not November of last year.

Speculation about iPhone release dates usually centers around the effect of holiday sales. The holiday retail season runs from October 1 through December 31, although some experts measure it from Thanksgiving through Christmas. Whichever calendar is used, iPhones are often given as gifts, or perhaps during the holiday season they are gifts people give to themselves.

Another theory about the September release date is that about a year between product launches is enough for people to want an upgrade. September launches began in 2012, so Apple has taken the iPhone through six years of this purchase cycle. The cycle also gives Apple’s product experts and engineers, along with major suppliers, time to gear up for a new model.

As an aside, among Apple’s primary worries should be that one year, the launch with the current interval won’t work. New features may not be sufficient for people to buy a new iPhone on a one-year interval. Some year, improvements to the iPhone may not be enough for many people to upgrade at all.

Regardless of what happens September 12, CEO Tim Cook will take the stage as Steve Jobs did before him. The tradition of the September iPhone launch will continue no matter what the results are for the upcoming year.