On the North American test realms:
Anub'arak - Tuesday Night, July 21, starting at 7pm EDT. Normal and Heroic.
On the European test realms:
Val'kyr – Wednesday Night, July 22, starting at 19:00 CEST. Normal and Heroic.
Blizzard is implementing both Normal and Heroic version in varying difficulties and set-ups. Daelodev also mentions having different Test Patchwerks that hits "VERY hard" to better test sustained tank damage. He reminds everyone that feedback from all the tests - which include their combat logs - is incredibly useful to the development team. He ends the post with the usual FAQ that notes that 1) there's no trash in the zone, 2) loot is implemented, and 3) Achievements are in place. Check out the official post for more details.