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:
I had the privilege of traveling to Irvine, California to play before the beta started and talk to the folks at Blizzard behind the project. They told a story of learning, enthusiasm, and cautious optimism in regards to WoW Classic, perhaps best articulated by J. Allen Brack, the recently appointed President of Blizzard Entertainment. He’s a thirteen-year Blizzard veteran who, until recently, had been with the WoW team since vanilla.
Most people who have spent years playing massively multiplayer online games will tell you that there's something special about the first one you played, too—and WoW was the first for many people. You can become so swept up in the uniqueness of your first experience that you overlook many flaws. It's likely many look at WoW Classic with rose-colored glasses, just as many simply forgot which features were added when. And some of today's WoW players may be too young to have even played vanilla WoW as it once was.