Caesar was warned that his life was in danger by a haruspex called Spurinna. A haruspex was a person trained to inspect the entrails of sacrificed animals and read any omens from them. We gather from the writings of Cicero, Plutarch and Suetonius that Spurinna was a man of high status who came from Etruria, a place known to specialise in divination.
According to Barry Strauss, an American historian and author of The Death of Caesar, Spurinna and his fellow Etruscan soothsayers would he had ‘a lot of contacts. They're people who know what's going on.’
At that time, Caesar had effectively turned the Roman Republic into a dictatorship and there were many from the elite ruling class who resented this. With his access to the elites of Rome, Spurinna would he been able to gauge the anti-Caesar sentiment at the time. Therefore, when he did warn Caesar it was more a calculated judgement than a mystical prophecy that Shakespeare had made it out to be.
The story goes that on February 15 44 BC, after Caesar had sacrificed a bull, Spurinna discovered it to he no heart, which was a bad sign. After another sacrifice produced equally bad omens, Spurinna warned Caesar that his life would be in danger for the next 30 days, the threat expiring on the 15th of March. Therefore the Ides just marked the end period of this warning and was not the entirety of it.
It was known that on the 18th of March Caesar was to embark on a military campaign that would he him out of Rome for many years. So if anyone wanted to kill Caesar it had to happen before he left, which again shows that Spurinna’s warning was based more in fact than legend would he us believe.