WoW: Classic is scheduled for August 27 (technically August 26 if you're in the Americas). The soonest that you can play it is starting in May, during Blizzard's scheduled stress tests. To opt-in you must have an active subscription on your Battle.net account. If you do, go to Account Management and select the Wow Classic beta. Keep in mind, there will be level caps during the test. The schedule is as follows:
A lot has happened in World of Warcraft in the fifteen years since it originally launched, and over that time we’ve come to expect certain things, not just in WoW, but in games in general. As players explore Azeroth as it existed back in 2006 during the WoW Classic beta, they’re reporting bugs – but in many cases, these ‘bugs’ are really just features that are working as intended.