The term 'MONTHLY OCCURRENCE' can be summarized as a regular event that happens once a month. The crossword solution for this term is 'IDES', which is an ancient Roman calendar system that divided the month into three parts: Kalends, Nones, and Ides. The Ides was the middle of the month, usually around the 15th day. This system was used to mark important dates and events, such as religious festivals and military campaigns.