Apple typically uses the first day of the show to host a keynote for developers and the media where it unveils its latest software and sometimes new hardware. That means it's expected to reveal iOS 13 for iPhone and iPad, as well as the latest versions of the software for Apple Watch, Apple TV and macOS for Macs.
Last year, Apple stuck to software and didn't announce any new hardware. But, in 2017, it used the opening keynote to reveal updates to the iMac, MacBook and MacBook Pro and also introduced the HomePod and a 10.5-inch iPad Pro.
Apple has already confirmed an event planned for March 25, where it's expected to announce new services, including streaming video and subscription news. If it doesn't announce any hardware at that event, it's possible we'll see new hardware — including AirPower, AirPods 2, the rumored 10.2-inch iPad and the high-end Mac Pro — on June 3.