Scroll down to see the testing schedule in the link, but here is a copy of the text:
*Classic WoW Testing Schedule
Beginning May 15, select WoW players will be invited to participate in a small-scale, focused closed beta test. Players will also get a chance to help put our servers and technology through their paces in a series of stress tests running from May through July—you can opt in now through Account Management and select the WoW Classic beta. Subsequent stress tests will extend the opportunity to even more players. Level caps will also be in place to ensure we’re emphasizing the “stress” in “stress test”
—we want to push our tech to the limit, and that means a critical mass of players in close proximity.
STRESS TEST SCHEDULE*
Stress Test 1: Wed May 22–Thurs May 23
Stress Test 2: Wed Jun 19– Thurs Jun 20
Stress Test 3: Thurs Jul 18– Fri July 19
Dates of each stress test are subject to change.