E3 or Bungie Day. Using math, I can prove that it will come out in fall 2013.
Taking the time between every major Bungie game since 2003, we can figure out that each game takes 3 years to make.
2001+3=2004+3+2007+3=2010+3=2013
Halo ---->H2 ---->H3 ---->H:R --->Destiny
Each game came out sometime in the fall. This means Destiny will come out in Fall 2013
[Edited on 02.26.2012 3:21 PM PST]