On your birthday, Apple will send a "Happy Birthday!" notification to your Apple Watch, and when you tap on it your watch face will be showered with celebratory balloons. The addition looks visually similar to the "Balloons" screen effect that launched in the iOS 10 Messages app last year. You can watch the full Happy Birthday animation in the video below, which was originally posted by David Boyle on YouTube.
Some of the big changes coming in watchOS 4 include a collection of Activity-focused updates with more personalized achievements for the Workout app, and communication features between Apple Watch and compatible gym equipment. There are also enhancements to the Music app, and a new Siri watch face that will dynamically update based on the time of day and your daily routines, and showcase important data from apps like Activity, Alarms, Breathe, Calendar, Maps, Reminders, Wallet, and more.