This was added in 1.9:
Raid Calendar System
The raid-lockout system has undergone a significant change. The new system will have all instances reset at a certain server time, regardless of when you were actually saved to the instance. The reset schedule is as follows:
- Molten Core: Every 7 Days, resetting during weekly maintenance.
- Blackwing Lair: Every 7 Days, resetting during weekly maintenance
- Onyxia: Every 5 Days
- Zul’Gurub: Every 3 Days
- Temple of Ahn’Qiraj (40-man): Every 7 Days, resetting during weekly maintenance
- Ruins of Ahn’Qiraj (20-man): Every 3 Days
Or is everything resetting once a week like Retail?