What are the ides of March?

Julius Caesar is not so much a tragedy about the title figure as it is about Marcus Brutus and how a good person can do bad things. When the play starts, Julius Caesar is already a military and political hero, loved by the people but hated by his political rivals.  When a fortune-teller accosts Caesar and warns him to "Beware the Ides of March," he ignores the plea. But his assassination on the Ides of March — March 15 — has forever imbued the date with dread.
Truthfully, in Roman times, the expression "Ides of March" did not evoke a sense of foreboding — it was simply the way to say "March 15." You might think that such a fanciful expression must signify something more than just another day, but not so. Even in Shakespeare's time, 16 centuries later, audiences attending Julius Caesar would have understood the expression.