Per Wikipedia: http://en.wikipedia.org/wiki/Ides_of_March

The Ides of March (Latin: Idus Martii or Idus Martiae) is a day on the Roman calendar that corresponds to 15 March. It was marked by several religious observances, and became notorious as the date of the assassination of Julius Caesar in 44 BC. The death of Caesar made the Ides of March a turning point in Roman history, as one of the events that marked the transition from the historical period known as the Roman Republic to the Roman Empire.[1]

Although March (Martius) was the third month of the Julian calendar, in the oldest Roman calendar it was the first month of the year. The holidays observed by the Romans from the first through the Ides often reflect their origin as new year celebrations.

The Romans did not number days of a month sequentially from the first through the last day. Instead, they counted back from three fixed points of the month: the Nones (5th or 7th, depending on the length of the month), the Ides (13th or 15th), and the Kalends (1st) of the following month. The Ides occurred near the midpoint, on the 13th for most months, but on the 15th for March, May, July, and October. The Ides were supposed to be determined by the full moon, reflecting the lunar origin of the Roman calendar. On the earliest calendar, the Ides of March would have been the first full moon of the new year.[2]

[1] ^ “Forum in Rome” entry in Oxford Encyclopedia of Ancient Greece and Rome (Oxford University Press, 2010), p. 215.

[2] ^ H.H. Scullard, Festivals and Ceremonies of the Roman Republic (Cornell University Press, 1981), pp. 42–43.