Blizzard devs will be adding lots of World of Warcraft Classic players ahead of the first stress test event that will take place on Wednesday, May 22nd from 4:00 - 6:00 pm Pacific / 7:00 - 9:00 pm Eastern. The Classic closed beta realm will be offline during the stress test, but CBT players can log into the special realm being set up for the stress test. Those who wish to participate will need to have opted in to Classic beta testing via their account profile.
If selected, the BattleNet launcher will say "Beta & Stress Test WoW Classic" in the drop down menu. At that time, players can download the client.
The action will all take place in starter zones and players will be able to level up to 5. While the stress test realm is technically a PvP realm, starter areas are considered "sanctuaries" and there will be no PvP taking place. "We expect to put a lot of people in the starting zones, though, and we're looking forward to seeing what you all do as we test the servers under crowded conditions."
The stress test events are being conducted as they contribute "to a smooth launch on August 27th".
Check out the full post on the World of Warcraft Classic forum.