A more thorough explanation:

Definition: Ides (Idz), n. [from Latin idus] (plural). In Roman law, the ninth day after the Nones, which falls on the 15th of March, May, July, and October, and the 13th of the other months. The first day counted in the calculation of the day is Nones.

Example: Julius Caesar was assassinated on the Ides of March, which is March 15th.

Explanation: This example illustrates the definition of Ides as the 15th day of March, which is the ninth day after the Nones. Julius Caesar was assassinated on this day, which has become famous in history as the Ides of March.