Blizzard have announced the release dates for each of the various types/wings of raid for the next patch, barring incidents:
With the release of Patch 5.4: Siege of Orgrimmar, four wings of an expansive new raid will begin to open. The Normal and Heroic raid difficulties, Raid Finder wings, and the new-to-5.4 Flexible mode wings will each be available starting from different dates.
To access the new Raid Finder wings, you’ll need a minimum average Item Level (ilvl) of 496 or higher.
This is our current schedule based on the planned patch release date of September 10. If we determine any changes are needed, we’ll update this article.
September 10 – Patch 5.4 Release
Normal
Flex Wing 1 “Vale of Eternal Sorrows”
September 17
Heroic (Normal Garrosh kill required)
Flex Wing 2 “Gates of Retribution”
Raid Finder Wing 1 “Vale of Eternal Sorrows”
September 24
Raid Finder Wing 2 “Gates of Retribution”
October 1
Flex Wing 3 “The Underhold”
October 8
Raid Finder Wing 3 “The Underhold”
October 15
Flex Wing 4 “Downfall”
October 22
Raid Finder Wing 4 “Downfall”
The original post is here: http://us.battle.net/wow/en/blog/10782714/Siege_of_Orgrimmar_Raid_Schedule-9_4_2013
There is more information about the /flex raiding here: http://us.battle.net/wow/en/blog/10175200, but in essence it is a type of raid available for a flexible number of raiders from 10 to 25 and any number in between. Fights, etc, will automatically adjust depending on the number of raiders there. Gear will drop in a similar way to LFR, and be of a level between LFR and Normal.
