The Ides of March

There was a day known as the Ides (originally set to correspond with the full moon) in each month of the ancient Roman calendar, but only one of them has made it into popular culture. This day is the Ides of March, which happens to occur on the 15th of the month. Its fame is due to Shakespeare's references to it in Julius Caesar.
The Ides of March in 44 B.C. was the day a group of Roman senators assassinated Julius Caesar.

Definition: On the Ides of March (or March 15), 44 B.C., Julius Caesar was assassinated, at the foot of a statue of Pompey where the Senate was meeting.

See In the Steps of Julius Caesar for why Senate met in the porticus attached to the theater of Pompey that day.

Because of the assassination of Julius Caesar and the soothsayer's exchange about it with Julius Caesar, in Shakespeare's Julius Caesar tragedy, the Ides of March now signifies a fateful day.

Changing Significance of the Ides of March
Before the assassination of Julius Caesar, the Ides of March was:

* A division of the calendar based on the phases of the moon. In some months, the Ides is on the 15th, and in others, it is the 13th.
* March's Ides marked the beginning of the consular year, since the two annually elected Roman consuls took office on the Ides from c. 220 B.C. to 153, when they began to take office on the Kalends of January (what we call New Year's Day).

Already in the days following the assassination, the Ides of March held a special significance. Instead of writing "the assassination of Caesar", Cicero could assume he would be understood when he alluded to the assassination by reference to the Ides of March:

Idus Martiae consolantur.
The Ides of March are encouraging.
~ Cicero Letters to Atticus.14.4 (url = homepage.usask.ca/~jrp638/DeptTransls/CicLetters.html April 19, 44 B.C.)

Source of the Expression "Beware the Ides of March"
The source of the expression is the Julius Caesar play by Shakespeare. Here is the relevant passage:

Caesar:
Who is it in the press that calls on me?
I hear a tongue shriller than all the music
Cry "Caesar!" Speak, Caesar is turn'd to hear.

Soothsayer:
Beware the ides of March.

Caesar:
What man is that?

Brutus:
A soothsayer bids you beware the ides of March.
Julius Caesar Act 1, scene 2, 15-19

Examples:
Cicero Was Not One of the Ides of March Conspirators
Like the Ides of March conspirators, Cicero believed killing Julius Caesar was an act of liberation. Caesar's opponents claimed Julius Caesar was trying to reinstate the monarchy. The conspirators have been criticized for lack of foresight in not planning what would come next. Cicero, who, although he sympathized with the plot against Caesar, had not been not one of the knife-wielding assassins, went on to support Julius Caesar's heir, Octavian, the first emperor of Rome.
(Source: http://ancienthistory.about.com/b/2011/03/15/the-ides-of-march.htm)