Beware the Ides of March

Generally speaking, "Ides" is a term from the ancient Roman calendar. The Ides fell on the 15th day of March, May, July, or October or the 13th day of any other month. Thus, the Ides of March was the 15th day of March.

The term is probably infamous because Julius Caesar was assassinated on the Ides of March in 44 BC, famously retold in William Shakespeare's play Julius Caesar.

In modern times, it has come to mean a foreboding of doom.

Sources: Wikipedia, my dad