Shakespeare derived his information for Julius Caesar from an English translation of Plutarch’s Life of Julius Caesar, Life of Brutus, and Life of Antony. Plutarch, a Greek historian, lived from 46 AD to 120 AD and obviously believed in the supernatural phenomena he records in these three biographical essays. Here is an example from his Life of Julius Caesar:
But destiny, it would seem, is not so much unexpected as it is unavoidable, since they say that amazing signs and apparitions were seen. Now, as for lights in the heavens, crashing sounds borne all about by night, and birds of omen coming down into the Forum, it is perhaps not worth while to mention these precursors of so great an event; but Strabo the philosopher says that multitudes of men all on fire were seen rushing up, and a soldier’s slave threw from his hand a copious flame and seemed to the spectators to be burning, but when the flame ceased the man was uninjured; he says, moreover, that when Caesar himself was sacrificing, the heart of the victim was not to be found, and the prodigy caused fear, since in the course of nature, certainly, an animal without a heart could not exist. The following story, too, is told by many. A certain seer warned Caesar to be on his guard against a great peril on the day of the month of March which the Romans call the Ides; and when the day had come and Caesar was on his way to the senate-house, he greeted the seer with a jest and said: “Well, the Ides of March are come,” and the seer said to him softly: “Ay, they are come, but they are not gone.”
Shakespeare himself, however, found these anecdotes useful for dramatic purposes. For one thing, they obviously help to recreate the atmosphere of a time when superstition was science. More importantly, Shakespeare uses the phenomena reported by Plutarch, including Calpurnia’s prophetic dreams, to create a heavy sense of foreboding and expectation in his audience—which he fully intends to disappoint. The audience should be expecting the big climax to come when--after all their planning and after all the strange wonders--the conspirators finally kill Caesar. But the anticipated cathartic assassination scene does not come off. It actually “fizzles”--as it is intended to fizzle. Here are the stage directions.
They stab Caesar, Casca first, Brutus last.
The assassins are thoroughly confused and disorganized. The effect is anticlimactic. Shakespeare’s magic is always in his words, and he fully intended to have Antony’s funeral oration stir his audience as no awkward action on a little wooden stage could have done.
Antony’s speech is pivotal. It not only turns the story around, but it turns the audience around. They have been identifying with Brutus and Cassius--but now they are identifying with Antony and Octavius. After all, these two Romans represent the winning side; and the audience, like the Roman mob, has been strongly moved by Antony’s words. They want to see Brutus and Cassius defeated, even though they feel sympathetic to Brutus right up to his death.
The omens Shakespeare borrows wholesale from Plutarch are used to create a strong dramatic effect. But the modern viewer understands that they must all have had rational explanations. Calpurnia’s dreams especially seem to show her feminine intuition rather than being warnings from nonexistent deities. She sensed the truth about Caesar’s flatterers, and she saw their dire intentions enacted in her dreams.
Each omen that forms is there to clear up the larger themes, as in fate and misinterpretation of signs. Before he dies, each of Caesar nightmares/omens reminds us of his impending demise (the soon downfall). Each are like warnings of what might occur if Caesar doesn't change his behavior. All in all, the omens exist to imply the dangers of failing to precieve and analyze the details of one's world.