The Great Revelation is the day when vampires made themselves known to the world. Not to be confused with "The Great Revelation", a graphic novel that acts as a prequel to the HBO original series True Blood, the Great Revelation occurred in 2006, in conjunction with the advent of Tru Blood, and three years before the events seen in the series' premiere episode, "Strange Love", at the beginning of the series' first season.
The most important factor that led to vampires deciding to "come out of the coffin" was the invention of Tru Blood, a synthetic blood substitute created by Japanese scientists. After a number of debates, the vampire Authority decided in favor of integration into the human world. This allows vampires to Mainstream.
The world's reaction was mixed, from general acceptance of vampires in some countries to large scale protests in others, including the south.
Vampires already having trouble fitting into main society were thrown back into their cause even further when Russell Edgington disrupted a TBBN newscast, killing Jerry McCafferty on national television and declaring war. ("Everything Is Broken")