Today is the day that Apple will release iOS 18, the 2024 major update to the operating system for the iPhone. Apple previewed iOS 18 three months ago, and here is a link to my post on what seem to me to be the most interesting new features such as the new-and-improved Control Center, numerous improvements to Messages and Mail, and more. And it is not just the iPhone. Today, Apple will also release iPadOS 18 for the iPad, watchOS 11 for the Apple Watch, macOS 15 Sequoia for the Mac, and visionOS 2 for the Apple Vision Pro.
I don't believe that Apple has announced a release date for tvOS 18 for the Apple TV, so that could be available today or it could come in the next few weeks. [UPDATE: tvOS 18 was released today, as was HomePod software version 18.]
If you want to be an early adopter, Apple often releases updates around 10am Pacific / 1pm Eastern, so that is my best guess for the timing of today's release. [UPDATE: iOS 18 was released around 10:07am Pacific / 1:07pm Eastern.] Otherwise, your device should give you a notification at some point during the next week that you can update it.