The Ides of March is the 74th day in the Roman calendar, corresponding to March 15th. It was marked by several religious observances and was a deadline for settling debts. In the earliest calendar, the Ides of March would have been the first full moon of the new year. The Ides of each month were sacred to Jupiter, the Romans supreme deity, and the Flamen Dialis, Jupiters high priest, led the "Ides sheep" (ovis Idulis) in procession along the Via Sacra to the arx, where it was sacrificed. In addition to the monthly sacrifice, the Ides of March was also the occasion of the Feast of Anna Perenna, a goddess of the year (Latin annus) whose festival originally concluded the ceremonies of the new year. The day was enthusiastically celebrated among the common people with picnics, drinking, and revelry. One source from late antiquity also places the Mamuralia on the Ides of March.

In modern times, the Ides of March is best known as the date on which Julius Caesar was assassinated in 44 BC. Caesar was stabbed to death at a meeting of the Senate. As many as 60 conspirators, led by Brutus and Cassius, were involved. According to Plutarch, a seer had warned that harm would come to Caesar on the Ides of March. Suetonius and the historian Cassius Dio characterized the slaughter as a religious sacrifice, noting that it occurred on the Ides of March at the new altar to the deified Julius.

The phrase "Beware the Ides of March" has become famous thanks to William Shakespeares play Julius Caesar. In the play, a soothsayer attracts Caesars attention and tells him to beware the Ides of March. Caesar dismisses the warning, but is later assassinated on the steps of the Senate.

Overall, the Ides of March is a significant date in Roman history due to its religious and cultural significance, as well as its association with the assassination of Julius Caesar.