The test will run from 2:00 p.m. to 3:00 p.m. EDT on Sunday the 12th (today!). Like those before it, the test is open to those who have pre-purchased the game and characters will not have been deleted. As in previous stress tests, the team is advertising that this test is, true to its name, designed to stress the servers. Players might experience features not working as intended, and that's par for the course.
Have fun if you make it into the game, and remember that this will hopefully make the real deal a much smoother experience for everyone in a mere 13 days.