Of course you want to begin with the Bard. The quote comes from Act 1, Scene 2 of "Julius Caesar." Caesar is in a crowd when a seer calls out to him.
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.
Caesar fails to heed the words, but Brutus will ultimately be among the assassins who kill Caesar on the 15th, the ides – or day of the full moon – of March.