"Beware the Ides of March" is a quote taken from Shakespeare's "Julius Caesar," warning that something bad is going to happen. A soothsayer predicts Caesar's death and offers this warning in Shakespeare's tragedy.
The early Roman calendar was made up of 10 months beginning in March. At the time of this calendar, dates were expressed by phases of the moon, Kalends, Nones and Ides.
The Kalends was the new moon phase at the beginning of the month. The Nones phase was the quarter phase, usually appearing between the fifth and seventh days. The Ides was the full moon on the 15th of the month. The Ides of March is the Roman calendar's March 15.
In 46 B.C., Julius Caesar was granted dictatorship for his lifetime. Concerned with Caesar's growing power, a group of senators, including some of Caesar's closest friends, stabbed Caesar 23 times, killing him on the senate floor on March 15, 44 B.C.
In 1601, William Shakespeare's Julius Caesar depicted a soothsayer calling out to Caesar from a crowd to "beware the Ides of March." Caesar does not heed the soothsayer's warning and is stabbed to death halfway through the play.
In modern language, a warning to "beware the Ides of March" is a warning that something bad may happen. The phrase is often used as a warning not to trust peers, colleagues or friends.