Months of the Roman calendar were arranged around three named days and these were reference points from which the other (unnamed) days were calculated:
Kalends (1st day of the month).
Nones (the 7th day in March, May, July, and October; the 5th in the other months).
Ides (the 15th day in March, May, July, and October; the 13th in the other months).
According to Plutarch, a seer had foreseen that Julius Caesar would be harmed not later than the Ides of March and on his way to the Theatre of Pompey (where he would be assassinated), Caesar met that seer and joked, "The Ides of March are come", meaning to say that the prophecy had not been fulfilled, to which the seer replied "Aye, Caesar; but not gone." This meeting is famously dramatized in William Shakespeare’s play Julius Caesar, when Caesar is warned by the soothsayer to "beware the Ides of March." Julius Caesar was stabbed 33 times (three and thirty wounds) according to Shakespeare’s play (23 times, in real life).
Since then, the day has taken on a new meaning — a day when prophecies of doom are realized.