The Ides of March
You know the story of today already. Forget about that most forgettable movie with the same title.
In modern times, the term Ides of March is best known as the date on which Julius Caesar was killed in 44 B.C. Caesar was stabbed (23 times) to death in the Roman Senate by a group of conspirators led by Marcus Junius Brutus and Gaius Cassius Longinus. The group included 60 other co-conspirators according to Plutarch.
According to Plutarch, a seer had foreseen that 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 have come", meaning to say that the prophecy had not been fulfilled, to which the seer replied "Ay, 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."
And, of course, there's Shakespeare's rendition of the prophecy (Act I, Scene 2) and the Plutarch quote in Act 3, Scene 1:
[To the Soothsayer] The ides of March are come.
Ay, Caesar; but not gone.
March 15 is one of those days that writers, readers of Shakespeare, academics, historians, students of Latin remember. The rest of the world? Not so much.