Because of the assassination and the soothsayer's exchange with Julius Caesar about the dangers he faced in Shakespeare's Julius Caesar tragedy, the Ides of March now signifies a fateful day. Here is the relevant passage:
What a day! Maybe even a fateful day. Don't even get me started.....Caesar: Who is it in the press that calls on me? I hear a tongue shriller than all the music Cry "Caesar!" Speak, Caesar is turn'd to hear. Soothsayer: Beware the ides of March. Caesar: What man is that? Brutus: A soothsayer bids you beware the ides of March.
Julius Caesar Act 1, scene 2, 15-19