Source (us.battle.net)
“From Friday, April 20 at 12:01 p.m. PDT (noon), until Monday, April 23 at 10:00 a.m. PDT you’ll be able to log in, team up with friends, and play each of the five heroic classes to level 13 as you fight to save the world from the impending demonic invasion.”
Download the Diablo III Beta Downloader
* A Battle.net Account is required, Must attach a BattleTag™