Direct from Blizzard:
December 2, 2014
Highmaul opens with Normal and Heroic difficulties accessible.
December 9, 2014
Mythic difficulty and the first wing of Raid Finder (Kargath/Butcher/Brackenspore) unlock.
December 16, 2014
Raid Finder Wing 2 (Tectus/Twin Ogron/Ko’ragh) unlocks.
January 6, 2015
Raid Finder Wing 3 (Imperator Mar’gok) unlocks.
We should be raid-ready and prepared to go in 1 week from today!
-Blackclaws