The term 'THE 13TH OR THE 15TH' can be summarized as referring to the two days of the month that are known as the Ides. The Ides is a Latin term used to refer to the middle of the month in the Roman calendar, and is usually the 13th or 15th day of the month. In modern times, the Ides is not used for any particular purpose, but it is still recognized as an important part of the Roman calendar.