What investors need to know about how iPhone release dates affect Apple's sales

Apple CEO Tim Cook attends the premiere for season two of the television series "Ted Lasso" at Pacific Design Center in West Hollywood, California, U.S. July 15, 2021.
Mario Anzuoni | Reuters

For the past 10 years, Apple has released new iPhone models in the fall, kicking off what analysts call the upgrade cycle. It starts with a big December quarter, as customers decide whether the latest model is worth buying. Sales explode, then decline sequentially over the year, as the release of next year's iPhone approaches.

But the timing of the actual release can vary from year to year and have big implications for the tech giant's quarterly numbers.