Calendar of Romulus:
The months were divided into day markers that fell at the start of the month, the fifth or seventh day, and in the middle of the month. These 3 markers were called Calends, Nones and Ides.
Calends (Kalendae, Kalends) signify the start of the new moon cycle and was always the first day of the month. It is derived from the Greek word καλειν, “to announce” the days of the full and new moon.
Nones (Nonae) were known to be the days of the half moon which usually occur 8 days before the Ides.
Ides occurred on the 15th day of March, May, July, and October, and the 13th day of the other months. They are thought to have been the days of the full moon.
Each day was referred to by how many days it fell before the Calends, Nones or Ides. For example, March 11 would be known as “Five Ides” to the Romans because it is four days before the Ides of March (March 15).