This was the day Julius Caesar was killed. The "ides" in the Roman Calendar were the 15th day of some months and the 13th day of others.
The "Ides of March" have come to be known as a metaphor for impending doom. But not all has been bad with the term.
There was a band named the Ides of March that had a hit record with the song "Vehicle" in the early 1970's. Guns N' Roses also recorded a song titled "Ides of March".
The term is well-known and widely used today. When you hear someone utter those famous words you can harken back to Julius Caesar and his day of "impending doom".