From Diablo Wiki
Unicorn Day is a reference to May 9th, 2011, where Mike Morhaime announced, during the Activision-Blizzard conference call, and spoke of the Diablo III beta. The conference call was covered by diii.net with a live chat client opened, which exceeded 900 fans eager to hear about the Diablo III beta.
The term will likely be reused for upcoming special events, such as the start of the beta and the game's release.
Naming the Day
It is named "Unicorn Day" after the long, industrious journey of the unicorn into Diablo consciousness, due in part to the art controversy, Bashiok posting images of farting unicorns, fan speculation of a secret unicorn level, Flux spurring on such debate, and a host of other sources.
A feverish excitement for the beta test of Diablo III ensued, spawning multiple threads across both the diablo.incgamer's forums, the battle.net forums, major media websites and other fansites around the internet.
It was generally assumed that the "5,911 points" was in reference to the date of Unicorn Day, May 9th, 2011, where Blizzard announced that the beta was to begin in the third quarter of 2011.
- See the beta page to learn more about the Diablo III beta, including speculation and how to enter the random lottery for a beta key.