WordNet (r) 3.0 (2006):

crack of doom
    n 1: (New Testament) day at the end of time following Armageddon
         when God will decree the fates of all individual humans
         according to the good and evil of their earthly lives [syn:
         Judgment Day, Judgement Day, Day of Judgment, Day of
         Judgement, Doomsday, Last Judgment, Last Judgement,
         Last Day, eschaton, day of reckoning, doomsday,
         crack of doom, end of the world]