Short answers, pulled from the story.
The Ides of March is a day on the ancient Roman calendar corresponding to the 15th of March on the Gregorian calendar. It was one of three fixed reference points Romans used to count the days of the month, and it was originally tied to the full moon.
Julius Caesar was stabbed to death at a Senate meeting held at the Theatre of Pompey on the 15th of March, 44 BC. As many as 60 conspirators, led by Brutus and Cassius, carried out the killing, which triggered the final crisis of the Roman Republic.
A haruspex named Spurinna warned Caesar that harm would come to him on the Ides of March. Plutarch records the prophecy, and the biographer Suetonius names Spurinna as the seer. On the morning of the assassination, Caesar joked to Spurinna that the Ides had arrived without incident; Spurinna replied that they had come but were not yet gone.
The Ides of each month were sacred to Jupiter, and his high priest, the Flamen Dialis, led an "ovis Idulis" (Ides sheep) in procession along the Via Sacra to be sacrificed. The Ides of March also hosted the Feast of Anna Perenna, a new-year goddess, and the Mamuralia, a scapegoat ritual. In the later Imperial period it was observed as Canna intrat, marking the birth of the god Attis.
Caesar's death triggered a series of Roman civil wars. His adopted heir Octavian eventually defeated his rivals and in 27 BC took the name Augustus, becoming the first Roman emperor and ending the Republic. Octavian also executed 300 senators and equites on the Ides of March in 40 BC to avenge Caesar's death.
The phrase comes from William Shakespeare's play "Julius Caesar," in which a soothsayer warns Caesar with those words. Shakespeare dramatized an exchange recorded by Plutarch and Suetonius, in which the haruspex Spurinna delivered a similar warning to Caesar before the assassination.