When I was in grade school, I couldn't have cared less about history.  Now, I can't get enough.  From the Mayans, to Titanic, even Ancient Rome.  I remember being in 3rd or 4th grade when I learned that Julius Caesar was not that cartoon guy holding a pizza on a spear, but an actual person.

Idus Martias, translated as "The Ides of March" was a date on the Roman calendar, which falls on March 15th.  In Ancient Rome, a festival for Anna Perenna was held on the Ides of March, but it wouldn't be long before another event would leave the Anna Perenna festival in the shadows.

The most well known occurrence on the Ides of March was the assassination of Julius Caesar in 44 B.C.  Caesar was killed at the foot of a statue of Pompey where the Senate was meeting.  Before Caesar went to the theater of Pompey to attend the Senate meeting, he had been given advice not to go, but he didn't listen.

Due to the assassination, and the soothsayer's exchange with Julius Caesar about the dangers he faced in Shakespeare's 'Julius Caesar' tragedy, the Ides of March now signifies a fateful day. Here is the passage from 'Julius Caesar' Act 1, scene 2, 15-19 :

Caesar: 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.

Soothsayer: Beware the ides of March.

Caesar: What man is that?

Brutus: A soothsayer bids you beware the ides of March.

Before the historical assassination of Julius Caesar, the Ides of March was a division of the calendar based on the phases of the moon.  In some months, the Ides is on the 15th, and in others, it is the 13th. It's supposed to be on the day of the full moon.  March's Ides was almost like the Ancient Roman's Inauguration Day, as it marked the beginning of the consular year.  The two annually-elected Roman consuls took office on the Ides from c. 220 B.C. to 153.  From 153 B.C. the consuls began to take office on the Kalends of January (what we call New Year's Day).

Even immediately after the historical assassination of Julius Caesar, the Ides of March could be understood to easily refer to the assassination.  Roman leaders didn't have to say "the assassination of Caesar." He assumed that he would be understood just by reference to "the Ides of March."

So if someone today warns you to beware, you could take their advice, or do what Caesar did.  Personally, I think I'd heed the warning!