The Romans did not number days of a month from the first to the last day. Instead, they counted back from three fixed points of the month: the Nones (the 5th or 7th, nine days inclusive before the Ides), the Ides (the 13th for most months, but the 15th in March, May, July, and October), and the Kalends (1st of the following month). Originally the Ides were supposed to be determined by the full moon, reflecting the lunar origin of the Roman calendar. In the earliest calendar, the Ides of March would have been the first full moon of the new year.
Panel thought to depict the Mamuralia, from a mosaic
of the months in which March is positioned at the beginning of the year (first half of the 3rd century AD, from El Djem
, in Roman Africa
The Ides of each month were sacred to Jupiter, the Romans' supreme deity. The Flamen Dialis, Jupiter's 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. This observance, which has aspects of scapegoat or ancient Greek pharmakos ritual, involved beating an old man dressed in animal skins and perhaps driving him from the city. The ritual may have been a new year festival representing the expulsion of the old year.
In the later Imperial period, the Ides began a "holy week" of festivals celebrating Cybele and Attis, being the day Canna intrat ("The Reed enters"), when Attis was born and found among the reeds of a Phrygian river. He was discovered by shepherds or the goddess Cybele, who was also known as the Magna Mater ("Great Mother") (narratives differ). A week later, on 22 March, the solemn commemoration of Arbor intrat ("The Tree enters") commemorated the death of Attis under a pine tree. A college of priests, the dendrophoroi ("tree bearers") annually cut down a tree, hung from it an image of Attis, and carried it to the temple of the Magna Mater with lamentations. The day was formalized as part of the official Roman calendar under Claudius (d. 54 AD). A three-day period of mourning followed, culminating with celebrating the rebirth of Attis on 25 March, the date of the vernal equinox on the Julian calendar.