Taken from Arena.net:

"We were very pleased with how our servers performed during our second Beta Weekend Event, but we want to gather additional data on how they operate under load before the next one.

That’s where you come in.

All players who have pre-purchased Guild Wars 2 are invited to participate in our upcoming stress test. This four-hour test event will run on Wednesday, June 27th, from 10 a.m. to 2 p.m. PDT (GMT-7).

We won’t wipe characters for this event, so if you played in our previous betas, you can pick up right where you left off. All we need you to do is log in and have fun—leave the stress to us!"