Last year Apple launched the iPhone 12 and 12 Pro (starting pre-orders in mid-October), followed by the iPhone 12 mini and 12 Pro Max (in early November). This year Cupertino may launch all four models simultaneously and at an earlier date too, based on evidence spotted in an ecommerce app in China.
The following screenshot was posted by @PandaIsBald and was reported on by IT Home. It shows that all four iPhone 13 models will go on sale on September 17. This is for China, but the country should be in the first wave.
The new Apple AirPods 3 TWS headset will arrive a couple of weeks later on September 30. Naturally, there will be a short pre-order period preceding the sales.
This information about the launch may be inaccurate (or even faked), though most analysts believe that Apple is on target for a September launch. And it is expecting the new models to sell better than the 2020 generation, Bloomberg reported that the company wants to have 20% more units available at launch.
Last year the pandemic caused a number of unexpected delays, which threw off the timing. Apple and its partners have learned to navigate the situation, so things should be back to the usual September launch schedule.