The anniversary bash kicks off with an address by President Mike O'Brien, followed by a sneak peek play session of exclusive new content, a panel on the living world of the game, a PvP invitational tournament, and a fun party. It is first-come, first-served and the event has limited capacity.
Can't come? Don't worry: ArenaNet will be livestreaming the event of anyone who would like to attend virtually.
[Thanks to Paul for the tip!]