Next week, on September 10th, we get normal mode opening and Flex Raiding opens wing 1, the Vale of Eternal Sorrows. Heroic opens on the 17th (requiring a Garrosh kill on normal to unlock it), along with Flex wing 2, the Gates of Retribution, and LFR opening wing 1. This keeps the previous intention of unlocking Flex faster than LFR and staggers the normal and heroic releases as we saw in previous raids this expansion.
For the complete release schedule, we've reproduced the Blizzard post behind the jump. I for one am very much looking forward to the Siege.