The Ides of March are the day of the warning and of the murder: a soothsayer warns Caesar about them in Act I, scene 2, and it is on that day, in Act III, scene 1, that he is killed in the Senate. In the Roman calendar, the Ides of March fell on the 15th.
In the Lupercal procession (Act I, scene 2), the soothsayer shouts that Caesar should beware the Ides of March. Caesar has him brought near, hears the same phrase again, and dismisses him: he is a dreamer. In Act II, scene 1, Brutus asks Lucius whether the next day is not the Ides and tells him to look at the calendar; Lucius returns and says March has already used up fifteen days.
In Act III, scene 1, on his way to the Capitol, Caesar tells the soothsayer that the Ides of March have come, and he replies that they have come, but have not yet passed. Shortly afterward Caesar is killed.
The date returns after the death. In Act IV, scene 3, Brutus tells Cassius to remember March, the Ides of March: if Caesar bled for justice, none of those who wounded him can now soil their hands with bribery. In Act V, scene 1, before the battle, Brutus says that this same day must end the work that the Ides of March began.