Raid lockout

Raid timers are the technique implemented by Blizzard to prevent members of large guilds from visiting high-end instances over and over to repeatedly farm bosses in a short period of time. Despite the name "raid" it also affects heroic dungeons.

Players re-entering a time-limited instance will find that they are entering the same place, regardless of whether anyone else is still in the instance; bosses killed in previous visits to the same instance stay killed until the timer resets. The other implication is that the players associated with the instance have a time limit in which to complete their activities before the instance is reset. This time can be extended (from Patch 3.2.0) with the Instance Lock Extension feature.

Bear this in mind if you are revisiting an instance with different raid members, as they might not always be "saved to" (associated with) the same instance as you. If you haven't got an ID for a certain instance, and you join a group with an ID for that instance, you will be saved to their ID as you enter the instance

Raid timers (or Raid IDs) are the system used to save you to a raid instance. To see which raids you are currently saved for, you can type /raidinfo This will give you a number of IDs, which you can then compare with others to see if you're saved to the same raid.

You may join raids that are saved if you are not saved- however, you cannot save yourself to another raid, even if it is similarly progressed.

Implications
If you are organizing a raid, it's best to enter with one other person on the same Raid ID as you, and then send invites from inside the raid instance. There have been problems experienced with raid timers being swapped, and people entering the wrong instances.

Instance timers
Since Patch 1.9.0, instances are reset according to a fixed calendar. This was decided by Blizzard to be fairest to most and replaces the previous system of resetting raid instances based on the absence from the instance of the raid members.

Unless a raid has been extended, the following is a list of when each raid is reset:

Classic raids

 * Zul'Gurub: Every 3 days at 3:00AM
 * Molten Core: Every Tuesday at 3:00AM or during weekly maintenance
 * Blackwing Lair: Every Tuesday at 3:00AM or during weekly maintenance
 * Onyxia's Lair: Every 5 days at 3:00AM
 * Ruins of Ahn'Qiraj: Every 3 days at 3:00AM
 * Temple of Ahn'Qiraj: Every Tuesday at 3:00AM or during weekly maintenance
 * Naxxramas: Every Tuesday at 3:00AM or during weekly maintenance

Burning Crusade raids

 * Karazhan: Every Tuesday at 3:00AM or during weekly maintenance
 * Gruul’s Lair: Every Tuesday at 3:00AM or during weekly maintenance
 * Magtheridon: Every Tuesday at 3:00AM or during weekly maintenance
 * Tempest Keep: The Eye: Every Tuesday at 3:00AM or during weekly maintenance
 * Coilfang Reservoir: Serpentshrine Cavern: Every Tuesday at 3:00AM or during weekly maintenance
 * CoT: Battle for Mount Hyjal: Every Tuesday at 3:00AM or during weekly maintenance
 * Black Temple: Every Tuesday at 3:00Am or during weekly maintenance
 * Zul'Aman: Every three days at 8:00AM
 * Sunwell Plateau:: Every Tuesday at 3:00AM or during weekly maintenance

Wrath of the Lich King raids

 * Naxxramas: Every Tuesday at 3:00AM or during weekly maintenance
 * Obsidian Sanctum: Every Tuesday at 3:00AM or during weekly maintenance
 * Vault of Archavon: Every Tuesday at 3:00AM or during weekly maintenance
 * Ulduar: Every Tuesday at 3:00AM or during weekly maintenance
 * Trial of the Crusader: Every Tuesday at 3:00AM or during weekly maintenance
 * Onyxia's Lair: Every Tuesday at 3:00AM or during weekly maintenance
 * Icecrown Citadel: Every Tuesday at 3:00AM or during weekly maintenance

Heroic Dungeons

 * Heroic dungeons are treated as Daily Raids and are reset every morning at 9:00AM (7:00AM EU), regardless of when bosses were killed.