You can join the war effort in WoW Classic when The Gates of Ahn’Qiraj open July 28.
In a new post by Blizzard, Community Manager Kaivax shares some additional details. It looks like Version 1.13.5 is currently in the final touches stage of polish, and will be live sometime in July. Included in that patch will be the content the team has been testing in PTR.
To that end, Ahn’Qiraj is set to unlock on July 28, coinciding with the raid reset for all realms,
“At that time, players can begin the quest chain to craft a Scepter of the Shifting Sands, and players can turn in gathered resources to advance the Ahn’Qiraj war effort.”
As soon as you’ve completed those activities, you can open the Gates of Ahn’Qiraj. Good luck!