Anytime from a week before to the morning of the event, if Apple is planning to stream the keynote (they are), they'll post a link on the homepage. If you have a Mac or an iOS device running any recent version of OS X or iOS, you'll be able to watch it there in Safari. Apple TV second generation and later will also show an "Apple Events" app with the WWDC 2015 artwork, which will allow you to stream the keynote live. As mentioned, you can also use the WWDC app on iOS devices to stream, if I remember correctly.
If you're on Windows or another OS and don't have any of the above devices, someone on the forums usually tracks down a URL the morning of the event which you can open in the VLC media player to stream it live almost as if you were on an Apple device. Keep your eyes peeled on the forum comments on front page posts before 10 AM Pacific June 8 if that's your situation.