The event will take place from Friday, April 27th, at 3:00 p.m. EDT until Monday, April 30, at 3:00 a.m. While there are various ways to obtain a key for the weekend (such as having gone to PAX East), the only 100% surefire way of getting in the event is to have pre-purchased the game. All event attendees will be emailed next week with instructions about the client download.
The beta event will allow players to choose from three races (Charr, Human, and Norn) and all eight classes, to experience PvP and PvE content, and to see Lion's Arch for the very first time. ArenaNet assured us that this will not be the final beta weekend prior to launch and delivered the good news that this event will not be hampered by an NDA of any sort.