The Ides of March -- what does that mean?

helpful definition and historical context (The Writer's Almanac, courtesy of Garrison Keillor) :

Today is the ides of March. The term 'ides' refers to an event in a lunar calendar; the 'ides' marked a full moon and noted the 15th of the month in March, May, July, and October, and the 13th in the other eight months. But when the lunar calendar became different than the monthly calendar, and the full moon was no longer always on the 13th or the 15th, the phrase went out of use.


Two thousand and fifty-five years ago on this day, in 44 B.C., the Roman emperor Julius Caesar was stabbed to death by senators who called themselves the Liberatores (Liberators) and claimed they were preserving the integrity of the Roman system. In modern usage, it was Shakespeare who popularized the phrase 'the ides of March' in his play Julius Caesar. Caesar is in front of a crowd of people, and he says, '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.' And a soothsayer replies, 'Beware the ides of March.'