Why should Caesar beware March 15th?

In Act I Scene 2, a soothsayer (a fortuneteller) three times warns Julius Caesar to “beware the ides of March.”  The ides indicate the middle of a month in the Roman calendar – the 15th day of March, May, July, and October, and the 13th of all the other months.  So the soothsayer is telling Caesar to beware the 15th of March.  Caesar dismisses him and does not heed his warning.  On the date in question, he tells the soothsayer in a mocking tone that the “ides of March are come,” to which the latter replies, “Ay Caesar:  but not gone.”  This is ominous, and sets the tone for the events that follow. 

All this talk of the ides of March is foreshadowing – the audience knows what is to come, and the dramatic irony of Caesar’s heedlessness has a very tense effect.  What the audience knows, and what Caesar does not, is that the 15th of March is the day Brutus and a group of other senators will have chosen to murder Caesar.  The soothsayer, who can read the future, is trying to warn Caesar to be alert; that on this date a threat will rise against him.  Yet Caesar scoffs at the man, and as often is the case with fate, any attempts to prevent it play directly into its hands.  So, Caesar must beware the ides because on the ides he will be murdered – and yet by warning him, the soothsayer has triggered his pride and his skepticism, and thus ensured that Caesar will fall into the trap fate has laid for him.