Where does the saying beware the Ides of March come from?

It is the day Julius Ceasar was assassinated (March 15th, 44BC)

The phrase was immortalized by Shakespeare in his play Julius Ceasar.

(Act I, Part II)

A soothsayer warns Ceasar to beware the Ides of March.

When the day comes he sees the seer again, and mockingly says

Well, the Ides of March are come.

she replies

Aye, they are come, but are not gone.

The Ides of March

The Ides of March has long been considered an ill-fated day. Julius Caesar was assassinated on March 15, 44 B.C. Historians note that it is likely that a soothsayer named Spurinna had warned Caesar that danger would occur by the ides of March. William Shakespeare included the phrase "Beware the ides of March" in his play Julius Caesar.

Each Roman Lunar Calendar month had three fixed named days, but was otherwise lacking a Julian numbering system as we're familiar with today.

# Kalends: #* Always the first day of the month. # Nones #* Always nine days before the Ides #* Long months (March, May, July and October) fell on seventh day of the month #* Other months, fell on fifth day of month # Ides: ## Always the day of the full moon in the lunar month. ## Long months (March, May, July and October) fell on 15th day of the month (remember the Ides of March) ## Other months, fell on 13th day of month The ides were the 15th days of long months (including Martius, or March) in the ancient Roman lunar calendar; they were the 13th in other months. The Word ides comes from the Latin Word idus, which is possibly derived from an Etruscan Word meaning "to divide." The ides were originally meant to mark the full Moon (the "halfway point" of a lunar month), but because the Roman calendar months and actual lunar months were of different lengths, they quickly got out of step. The ancient Romans considered the day after the kalends (first of the month), nones (ninth day before the ides, inclusive), or ides of any month as unfavorable. These were called dies atri.

ref: http://almanac.com

Dies Atri

dies atri - or "black days" were not marked on the calendar because they always occured on the day after the Kalends, Nones and Ides of each month. Nothing new could be done on dies atri, and even state cult festivals were not held on these days. Romans also thought that the Kalends, Nones and Ides of each month as well as the fourth day of each month were unlucky days (the way we would think of Friday the Thirteenth). There were no legal or religious prohibitions against activities on simply unlucky days, and a nundina could fall on Nones quite easily. Furthermore, Romans thought the month of May and the first half of June were terribly unlucky months to get married in.

ref: http://abacus.bates.edu