For today's Special Event, Apple will offer a live video stream you'll be able to watch on the Apple TV via dedicated "Apple Events" channel, and you'll also have the chance to watch it on your Mac (using Safari), or even on your iOS device if you have no access to any other device.
Apple has made the stream available on their "Apple Events" page.
The last time Apple has done this was about 3 years ago for the Apple Special Event on September 1, 2010, where Steve Jobs unveiled a new iPod lineup, Apple TV, and iTunes 10 – also taking place at the Yerba Buena Center for the Arts.
At that time they even released an official Press Info about it – one day before the event.
Something else Apple has done for the first time is to put a splash page about the event on their site, mentioning the event (might be better than a Press Info only Apple aficionados and journalists read).