This the the Roman Calendar for March. The Roman month had three key days: Kalends, Nones, and Ides. The Kalends originally designated the day of the new moon and was called the first of the month, the Nones designated the day of the half moon, and the Ides designated the day of the full moon.
The days of the month are referenced to the NEXT key day, so for example, the second day of March is ad VI Non Mar, meaning the sixth day before the Nones. The Romans counted inclusively so the key day itself is counted in the number. After the Nones the countdown starts to the Ides and then after the Ides the countdown starts to the next Kalends (first day of the next month). Once the Ides is passed, the days reference the next month rather than the current month, so the day after the Ides is sixteen days before the first of April.