Updated April 21st, 2020.
Watch the March 21st, 2016 keynote using your iPhone, iPad or Mac by clicking the big blue button.
Hint: if you’re using an iPhone or an iPad, be sure you’re on WiFi so you don’t use up your cellular data. These events usually run about two hours and you don’t want to burn through your monthly data allowance if you don’t have to.
If you have an Apple TV you can watch the keynote on your real TV. How you do it depends on which Apple TV you have.
If you have the 4th-generation Apple TV (see below) you’ll need to download the Apple Events app via the Apple TV App Store. (If you need help setting up the App Store part of your Apple TV, read this page from Apple— it tells you exactly how to do it.)
If you have a 2nd- or 3rd-generation Apple TV you’ll find an Apple Events app on your home screen automatically. Easy stuff.
You’ll be able to watch the keynote later in the day, or even later than that if you’d like. Apple maintains a webpage with recent keynote speeches, and they offer many others via this link to iTunes.
Since you asked: no, I don’t know what they will show during the keynote. Apple always drops a hint via the invitations they send out– this time, the event is titled “Let us loop you in.” To me, “loop” could refer to the Apple Watch’s watchbands (there’s a Leather Loop watchband, and a Milanese Loop watchband), or to Apple’s headquarters at 1 Infinite Loop, Cupertino California. Maybe there’s a new watchband coming out, and maybe there’s news about Apple’s new headquarters. Even if there’s both, that’s not enough for a special event. There must be something else… and you can bet I’ll be watching the keynote to find out!