The Ides of March is the day on the Roman Calendar corresponding to March 15. Originally the Romans celebrated the new year from March 1 to March 15, but it has more signifcance because of two historical events.
The first of course, was the assassination of Julius Caesar. He clashed with the Roman senate because he declared himself dictator for life and also illegally invaded Roman territory, instigating civil war. The outcome was Caesar controlling Rome.
A group of 60 Roman Senators led by Marcus Junius Brutus conspired to eliminate Julius Caesar. Scholars debate whether the dying Caesar exclaimed, "You, too, Brutus?"
And that brings up the second event in history for which we know the Ides of March. Without Shakespeare's Julius Caesar, the emperor would be far less known today. Believed to have been written in 1599, the play has Caesar only appearing in three scenes. It focuses instead, on Caesar's close friend, Brutus. In Shakespeare style, Brutus battles with his conscience and listens to the wrong people. Adding intrigue, Shakespeare has Julius Caesar warned by a soothsayer to beware the Ides of March, and also premonitions by Caesar's wife.
Brutus believes he has killed Caesar in duty to Rome, so he does not flee after Julius Caesar dies. Brutus battles his conscience once again, and commits suicide the next day.