Googling: "The Roman calendar, according to Britannica.com, divided months into groupings of days counted before certain named days. So, one grouping, the Kalends, was the beginning of the month. The Ides were the middle, and the Nones were between them. If a month had 31 days, like March, Britannica says, The Kalends would be day 1. Days 2-6 would be "before the Nones." The Nones would be day 7, Days 8-14 "before the ides" ...Which... leaves the 15th to be the "ides of March." What makes the day famous, obviously, is not just the Shakespeare play. Julius Caesar was actually assassinated on March 15 - the ides - of 44BC.