This is a Roman lunar calendar, based on observations of the phases of the moon. The Romans counted their days in a different way from us - instead of counting from 1-29 or 30, as we do - they had three fixed days in the month and counted backwards towards them. The first day of the month was called the Kalends (from which we get our word 'calendar' ) and was originally the day of the new moon. The nones was either the fifth or the seventh day, and was originally the day of the half moon; the Ides was the thirteenth or the fifteenth, and was reckoned to be the day of the full moon.