Synonyms for Doomsday (same or very similar meaning)
WordNet sense 1 ((New Testament) day of the Last Judgment when God will decree the fates of all men according to the good and evil of their earthly lives):
Judgment Day, crack of doom, Day of Judgement, Day of Judgment, day of reckoning, end of the world, Judgement Day, Last Day, Last Judgement, Last JudgmentWordNet sense 2 (an unpleasant or disastrous destiny):
doomFrom the ODE community, based on WordNetadd/edit