Today's word is not obsolete, it comes out of a question my brother asked me today. Is March the only month with ides.
Ides is the term for the 15th day of March, May, July, or October or the 13th day of any other month in the ancient Roman calendar.
The Ides of March are famous, of course, for being the day that Julius Caesar was assassinated. The phrase Ides of March has come to mean impending doom.