In Act II, Scene 1, Brutus asks Lucius:
Is not tomorrow, boy, the ides of March?
Lucius does not know, so Brutus sends him to look at the calendar. This appears to be Shakespeare's way of letting his audience know that tomorrow will be the day the Soothsayer warned Caesar about in Act I, Scene 2.
Beware the ides of March.
What man is that?
A soothsayer bids you beware the ides of March.
When Lucius returns and confirms that tomorrow is the ides of March, it is especially significant because Brutus was present to hear the Soothsayer's warning to Caesar. This should inform the audience that the great historical event being dramatized on the Elizabethan stage is about to take place. Brutus may feel that Caesar's assassination was predestined by the gods or by Fate. When Caesar hears the Soothsayer's warning in Act I, Scene 2, he does not take it seriously. He says,
He is a dreamer. Let us leave him. Pass!
Brutus appears to have remembered what the Soothsayer said and to treat it seriously. In Act II, Scene 1, he finds himself heavily involved in a conspiracy to assassinate Caesar, and the Soothsayer's warning must seem especially significant to him.
In Act III, Scene 1, which occurs on the ides of March, Caesar will encounter the Soothsayer again and show his disdain for the man's warnings—and prognostications in general—by saying, in effect, "The ides of March are come and I'm still here, as you can see for yourself." The Soothsayer's reply seems heavily ominous.
Ay, Caesar, but not gone.