The word Ides has Latin etymology that means "half division", which is logical as it occurs at almost mid-month of March. This day is often celebrated in honor of the Roman God, Mars and festivities include military parades and similar activities.
In popular literature and history, the Ides of March also pertains to the date when Julius Caesar was killed in 44 B.C. According to stories, he was stabbed 23 times in the Roman Senate and bled to his demise.
The term became related to that event as a seer foretold that Caesar will be "harmed not later than the Ides of March and on his way to the Theatre of Pompey," the place where he eventually was assassinated.
The exact term "beware of the Ides of March" was used and popularized by William Shakespeare in his play "Julius Caesar" where it appeared during Act 1, Scene II.
Soothsayer: Caesar!
Caesar: Ha! who calls?
Casca: Bid every noise be still: peace yet again!
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.
Caesar: Set him before me; let me see his face.
Cassius: Fellow, come from the throng; look upon Caesar.
Caesar: What say'st thou to me now? speak once again.
Soothsayer: Beware the ides of March.
Caesar: He is a dreamer; let us leave him: pass.