Quoted from an ArenaNet blog:
Everyone who pre-purchased Guild Wars 2 will enjoy three full days of Headstart Access. The three-day Headstart Access will officially begin at 0:00 AM Pacific time (GMT -7:00) on August 25th. However, please note that in order to ensure that we’re fully prepared for that fateful hour we may bring servers online up to 3 hours prior. If you’re a player committed to getting in first to grab that character name of your dreams you’ll want to be keeping an eye on things during that time period.
This is very, very important for those of you who plan to use potentially common names. Remember that names are game-wide and not by server, so make sure you’re spamming the login servers come 9PM on the 24th! With that being said, the vent gathering will start at 8:30 on the 24th, not 10PM. See you there!