
The two raid encounters to test out are the Imperial Vizier Zor'lok (Heart of Fear) and the Elegon (Mogu'shan Vaults) and are starting at 19:30 CET and 01:00 CET respectively.
Blizzard puts up a warning's finger that they might need to change the time of a testing session or cancel it entirely due to bugs, builds or server hardware issues. They also say that Elegon might get switched to Blade Lord Ta'yak.

We will be testing two 10-player Heroic raid encounters tomorrow, Thursday July 26th.
Each encounter should be available at approximately the listed times below for all Beta Test Realms, regardless of suggested geographical region.
Imperial Vizier Zor'lok (Heart of Fear) - 10 Player Heroic
10:30 PDT (13:30 EDT, 19:30 CEST)
Elegon (Mogu'shan Vaults) - 10 Player Heroic*
16:00 PDT (19:00 EDT, 01:00 CEST)
As always, this testing schedule is very fluid and subject to the realities of a beta environment. We might have to change the time of a testing session or cancel it entirely, due to bugs, builds, server hardware issues, etc. Keep an eye on this forum for the latest information, and thank you in advance for testing and providing feedback.
*In particular, Elegon may change to Blade Lord Ta'yak instead. We'll update this post accordingly.