Although the earliest Roman calendar was lunar, its year had only ten months and 303 days. In the eighth century BCE, two more months were added. The calendar did not depend upon astronomy relating to agriculture, but on the declarations of priests who observed movements in the sky. If a priest did not see the beginning of the moon’s crescent, the announcement of a new month would be delayed. He balanced the year by adding a day three times in eight years.
About 700 BCE, Numa Pompilus added January and February to the old calendar. The year was now based on 365 days, with four months having thirty-one days, one having twenty-eight days, and seven having twenty-nine days.
The month was divided into three unequal sections, with the first day of them being Kalends, Nones, and Ides. Days were numbered according to the sections.