There was a day known as the Ides in each month of the ancient Roman calendar, but only one of them has made it into popular culture. This day is the Ides of March, which happens to occur on the 15th of the month. Its fame is due to Shakespeare's references to it in Julius Caesar.
The Ides of March in 44 B.C. was the day a group of Roman senators assassinated Julius Caesar.
Read about the Ides of March.

Julius Caesar

Another event on This Day in Ancient History was the festival for Anna Perenna, Roman goddess of the New Year.