I knew Caesar was asssassinated on the Ides of March, and that that was the 15th of march, but today I've finally found out that each month has an 'ides', but it's not always on the 15th.Il s'assemblait régulièrement trois fois par mois, aux calendes, aux nones et aux ides.
Roman dates were counted inclusively forward to the next one of three principal days within each month:
Kalends (Kalendae or Kal.), the 1st day of each month
Nones (Nonae or Non.), the 7th day of full months and 5th day of hollow ones, 8 days—"nine" by Roman reckoning—before the Ides in every month
Ides (Idus, variously Eid. or Id.), the 15th day of full months and the 13th day of hollow ones, a day less than the middle of each month
These are thought to reflect a prehistoric lunar calendar, with the kalends proclaimed after the sighting of the first sliver of the new crescent moon a day or two after the new moon, the nones occurring on the day of the first-quarter moon, and the ides on the day of the full moon. The kalends of each month were sacred to Juno and the ides to Jupiter. The day before each was known as its eve (pridie); the day after each (postridie) was considered particularly unlucky.
... The four 31-day months were called "full" (pleni) and the others "hollow" (cavi).
https://en.wikipedia.org/wiki/Roman_calendar#Days