When World of Warcraft: Classic launches this summer, it will be parsed out into six phases, up from the original four when it was announced. The decision to divide it differently came as a result of inner "deliberations" and based on player feedback. "Were aiming to capture what it felt like to play in a realm community in original WoW. To do that, we're planning to mirror the approach taken by original WoW through patch 1.2.x," the latest community update reads.
Content will roll out starting with Onyxia's Lair, Maraudon and Molten Core. Dire Maul will come at a later point due to the loot that's available that "would affect progression through those early raids". Kazzak and Azuregos will also come at a later point for the same reasons.
Blackwing Lair and Zul'Gurub will not unlock at the same time since that would be different than they were originally released. "It makes sense to not have gear and enchants from Zul'Gurub available during early progress into Blackwing Lair". Emerald Dragons will arrive before the opening of Ahn'Qiraj.
Here’s what the release order currently looks like: Phase 1 (Classic Launch)
Molten Core Onyxia Maraudon
Dire Maul Azuregos Kazzak
Blackwing Lair Darkmoon Faire Darkmoon deck drops begin
Zul’Gurub Green Dragons
Ahn’Qiraj War Effort begins Ahn’Qiraj raids open when the war effort dictates Dungeon loot reconfiguration: Tier 0.5 Dungeon gear, Relics, drop rates and location changes
Naxxramas Scourge Invasion
The list above is missing PvP as "we're still evaluating our options regarding PvP rewards, as they also changed over time".
Read the full post on the World of Warcraft Classic forum.