In ancient Rome, the calendar year began on March 15. The 15th of each of month was known as the ides, from a Latin word that indicates division of a month. At any rate, back in 44 b.c., the legendary emperor Julius Caesar summoned members of the Senate to meet in the Theatre of Pompey on March 15. Previously, a soothsayer had warned Caesar to "beware the ides of March," but since not much had happened that day, Caesar felt confident attending a Senate session. After all, the men of the Senate were loyal to him, so how much harm could he possibly come to?
Unfortunately, Caesar had enemies within his own Senate, and upon his arrival, several members of a group known as the Liberatores fell upon him and stabbed him to death. Some notable names were in the group, particularly Gaius Cassius Longinus and Marcus Junius Brutus, the son of Caesar's lover. They claimed that their actions were not treasonous, but in fact tyrannicide.
In 1599, when Shakespeare wrote his famous play, he made sure to include the seer's cautionary line about being wary on the date, and thus the term "beware the ides of March" has come to bring about a sense of impending doom. He also attributed an equally famous line to the dying emperor, "Et tu, Brute? Then fall, Caesar."