noted on a blog post is intended to celebrate the achievement of more than 1.5 million players completing Act 5.
Blizzard held another 50 percent bonus XP event last month, though that was intended to brace the world for Death's coming. Now that we've had bonus XP to prepare for his arrival and celebrate his defeat, it'll be interesting to see what justification Blizzard gives should another weekend of bonus XP occur - Death's sister's boyfriend's aunt celebrating a half-birthday, perhaps?
Unfortunately, only nephalem with access to a computer of either the Mac or PC variety can partake. Sorry, console-bound adventurers.