In Act I Scene 2, the soothsayer says only one short line to Caesar, but he says it twice. The line is the famous saying, "Beware the Ides of March" (line 20). The Ides of March is March 15, so the soothsayer (a fortune teller) is warning Caesar that something bad will happen to him on that day. Caesar pays little attention to him. In fact, he couldn't even hear him at first, hence the reason why the soothsayer repeated himself. Caeser quickly dismisses him by saying "He is a dreamer. Let us leave him. Pass!" (line 25).
Though Caesar ignores the soothsayer, he ends up running into him again in Act III, Scene I. Caesar remembers the Soothsayer's warning and says, "The Ides of March are come" (line 1). Caesar is basically mocking the soothsayer because his warning didn't hold up. The Soothsayer replies, "Ay, Caesar, but not gone" (line 2). Of course, a few hours later, Caesar is killed and the soothsayer is vindicated.
In this scene all of Rome is celebrating the Feast of Lupercal, a fertility festival held in honor of the god Lupercus, or Pan; as part of the festivities a foot race is held, in which Marc Antony participates. So, needless to say, there is a very large crowd around Caesar, out for this popular festival. In the throng, the soothsayer calls to Caesar, who, hearing his voice, bids him approach and speak. “Beware the ides of March,” is all he will repeat—a warning of what he has seen in his fortune-telling. Caesar, however, does not take the warning seriously, and instead dismisses the man immediately, stating that “He is a dreamer. Let us leave him. Pass.”
The story of Julius Caesar is well-known today, and was perhaps more well-known in Shakespeare’s time. Thus this event is an example of dramatic irony—the audience knows of Caesar’s fate, and yet Caesar himself disregards the only warning he receives of his forthcoming murder. Similarly, later in the scene Cassius hints to Brutus of his plans to assassinate Caesar, and Caesar, speaking with Antony, notes how he mistrusts Cassius—he “has a lean and hungry look; / he thinks too much. Such men are dangerous.” And yet he asserts, “I fear him not.” Here again is irony, for indeed, if Caesar has anyone to fear, it is Cassius.
The soothsayer calls out Caesar’s name and Caesar responds by asking who called him. Casca asks the others to remain quiet and Caesar asks again, “Who is it in the press that calls on me?” The Soothsayer responds back to Caesar and informs him to beware the ides of March. Caesar is curious to know who issued the warning and asks him to come forward. Brutus informs Caesar that it is a soothsayer, Caesar asks the soothsayer to speak again and the soothsayer repeats the phrase “Beware the ides of March”. Caesar scoffs at the soothsayer and calls him a dreamer. In essence the soothsayer is warning Caesar of his demise, specifically the assassination that will be executed against him. Caesar, on the other hand, does not heed this warning and believes in his authority.
In Act III, while Caesar heads to the Capitol at the Senate sitting, he tells the soothsayer that the ides of March are come, meaning that nothing has happened to him. The soothsayer however responds that the ides of March are not gone, meaning the day is not over yet. Caesar ignores the soothsayer again and walks straight to his assassination.