in the Roman calendar the Ides fell on the following days: January 13, February 13, March 15, April 13, May 15, June 13, July 15, August 13, September 13, October 15, November 13, December 13; often interest, debts, and tuition were paid on the Ides.
In Rome, Ides was originally the day of a full moon. By the fifth century B.C. its date was fixed as the 15th day of March, May, July & October and the 13th day in other months.