The kalends was always the first of the months, the ides was the middle of the month, and the nones was in between. Until March 1st, 4 С.E., the Roman calendar system has changed frequently (see the brief history of the calendar for the calculator), so do not rely on an exact date matching up to this point. Over time, the calendar was aligned with the solar year, and the original significance of these events was lost, but the kalends, ides, and nones remained an integral part of the Roman calendar. Initially, the kalends coincided with the new moon, the nones - with the first quarter, ides - with the full moon. Similar to other ancient solar calendars, the Roman calendar was inherited from a similar lunar calendar.
All other days of the month were identified by counting days up to one of three events, e.g., 10 days before the Kalends of June.
So three days of the month were named after these events, e.g., Ides of March or Nones of April or Kalends of May. The Roman calendar had 3 special monthly events: calends, nones and ides.
The Roman calendar's unusual feature is a day identification by inclusive counting up to a coming month event.
