Today March 15, is the infamous Ides of March, the day Julius Caesar was assassinated by a group of Roman Senators led by Brutus and Cassius.  The murder became the subject of one of Shakespeare’s most famous plays. Consequently, most high school students readily remember the eerie prediction of the Soothsayer in the great tragedy  “beware the Ides of March”.  Interestingly, in the Roman Calendar, the Ides does not follow on the 15th of every month. In fact there are only three months in which this applies…March, May July and October.

But here is a more detailed explanation on how the Ides was determined in the Roman Calendar : (http://ancienthistory.about.com/cs/calendars/a/romcalterm.htm).

The Roman calendar was originally based on the first three phases of the moon, with days counted, not according to a concept of week, but backwards from lunar phases. The new moon was the day of the Kalends, the moon’s first quarter was the day of the Nones, and the Ides fell on the day of the full moon. The Kalends’ section of the month was the longest, since it spanned two lunar phases, from the full to the new moon. To see it another way:

• Kalends = New Moon (no moon to be seen)
• Nones = 1st quarter moon
• Ides = Full Moon (whole moon visible in the night sky)

When the Romans fixed the length of the months, they also fixed the date of the Ides. In March, May, July and October, which were (most of the) months with 31 days, the Ides was on the 15th. On other months, it was the 13th.