Predictions based on
Machine learning guesses.
iOS releases are always a great time to market your app: Support the new tentpole iOS features early and get on Apple's radar (e.g. support for Widgets, Shortcuts, HealthKit…).
A major macOS release is a great time to get attention. Everyone visits the Mac App Store to download the update, so aim to get some visibility.
Even non-developers pay attention to WWDC. The biggest opportunity any app has is to win a Developer Award – start talking to Apple a few months before to be in with a chance.
If you can support the latest & greatest hardware features in time for launch day, let Apple know for one of the coveted feature slots.