In Act 1.2, the Soothsayer (one who predicts the future) twice warns Julius Caesar, "Beware the ides of March." He has foreseen that on this date, March 15, Caesar will meet a grim fate. Caesar does not listen to him, nor to the warnings of his wife, Calpurnia, who also senses that danger looms. Caesar's decision to ignore both the Soothsayer and Calpurnia speaks to his hubris (excessive pride, presumption and/or arrogance) will be his undoing. Though he "doth bestride the narrow world like a Colussus), he is also "spoiled by success and adultion" (Homer Watt).
Here are the lines in which he spurns the Soothsayer (1.2.25-26):
Beware the ides of March.
He is a dreamer; let us leave him: pass.