The 15th of March in the Roman Calendar. Got its bad rep in 44 B.C., when Brutus killed his buddy (and the emperor of Rome) Caesar. Immortalized and made to sound foreboding in Shakespeare's portrayal of the event and the prophesies leading up to it.
If you're planning on being betrayed, act now! This is the most dramatic time to do it, other than, say, your wedding day.