It is 44 BC and Julius Caesar is at the height of his power and popularity. He has defeated his rival Pompey, and the Roman citizens are preparing to celebrate the feast of Lupercal. However, Caesar does not know that there is a conspiracy afoot to assassinate him and restore the ancient republic. As he moves through the cheering crowds, a voice is heard saying, "Beware the Ides of March!" (the 15th.) Caesar takes no notice. Shakespeare makes this event (which probably did take place historically) full of dramatic irony as, immediately afterwards, we are shown Brutus, his trusted friend, discussing the possibility of a republic with Cassius and Casca, two men who are envious of Caesar's fame. In fact this pattern is repeated; later, Caesar's wife has a dream that he is in danger and gives him another warning, which he again ignores. On the 15th March, he is assassinated.
Each period of the year was divided into smaller periods, and each month too. The Ides were the middle of a month. Caeser was warned to beware of the Ides of March because a soothsayer saw him being killed in this period.