"Beware the Ides of March!"
This was a warning to Julius Caesar by the soothsayer in William Shakespeare's 'Julius Caesar'. This was the day that the Caesar was to meet his untimely demise at the hands of Brutus and the other disgruntled politicians.
In Roman times the expression "Ides of March" did not necessarily evoke a dark mood—it was simply the standard way of saying "March 15." Surely such a fanciful expression must signify something more than merely another day of the year?
For a further history lesson, read this article.