Battle for Azeroth Mythic dungeons will first become available after the first weekly reset -- the morning of August 14 in the Americas region, August 15 in Europe, etc.
We’ve seen some confusion about when Battle for Azeroth’s first Warfront, Battle for Stromgarde, will open and would like to give some clarifications.
Similar to other types of endgame content such as raids and rated PvP, the Battle for Stromgarde will not be immediately available at launch. Instead, it’ll open a few weeks later. Initially, the Alliance will control Stromgarde Keep (which allows access to certain outdoor content in the Arathi Highlands), while the Horde begins preparations to assault. Once those preparations are finished, Horde players will be able to queue for the Battle for Stromgarde Warfront.
A few days afterward, the roles will reverse: the Horde will control Stromgarde (thus gaining access to the outdoor content), and the Alliance will begin their own preparations to retaliate. This process will continue to alternate between Horde and Alliance on a regular cadence.