Three days of importance on the Roman calendar are the Kalends, the Nones, and the Ides of a given month. For four of the months (i.e., March, May, July, and October), the Ides fall on our 15th, and the Nones of those months on the 7th. For the other eight months, the Nones and the Ides were on the 5th and the 13th. The Kalends were always the first of the month. Dates were calculated by counting backwards from one of these three days (inclusively). See Wikipedia.

Ceci n'est pas un seing.