National Public Radio (NPR.org) is very good about remembering the Ides of March. This year, they ask the question why we never hear about the Ides of any other month...though listeners wondered if the question was answered sufficiently as the expert consulted mistakenly identifies the Ides as occuring on the 15th day of each month. (Edited to add: As it turns out, the expert did discuss how the Ides can be on either the 13th or 15th, but that part ended up "on the cutting room floor" and not in the portion that was broadcast on air.)
The Romans expressed their dates using the Kalends, Nones and Ides. The first day of each month was called the Kalend. In March, July, October and May, the seventh day was called the Nonae and the fifteenth, the Idus. In other months, the Nonae fell on the fifth day, the Idus on the thirteenth. To give dates, the Romans would count backward from these dates. Most Latin grammars (George Lane; Gildersleeve and Lodge; Charles Bennett) include a section on Roman calendar dating. To find out how the Romans would express a date that didn't fall on a Kalend, Nonae or Idus, consult one of these aforementioned reference grammars for more information. It's a bit complex, but not impossible to understand, and these grammars often include quick reference charts for determining dates. The point here is, however, that the Ides don't always fall on the fifteenth.
According to this rhyme from the 19th century grammarians Basil L. Gildersleeve and Gonzalez Lodge:
"In March, July, October, May,
The Ides are on the fifteenth day,
The Nones the seventh; but all besides
Have two days less for Nones and Ides."
Listen to Beware: It's the Ides of March on Npr.org to see what you think.
Since it's the Ides of March, and we're still thinking about Caesar and calendars, it's also worth mentioning that Caesar reformed the Roman calendar, by fixing the number of days in a year at 365, adding a leap day every four years. The Julian Calendar (named, of course, for Julius Caesar) was eventually itself reformed by Pope Gregory XIII in the 15th century. The calendar we use today is called the Gregorian Calendar.
Click here for links to previous NPR radio presentations referencing the Ides of March.
If you have a blog, you can add a Roman calendar to it. Visit Laura Gibb's Schoolhouse Widgets for details.