This would be a lot easier if the year ran from March 1 to February 28/29. Which it did, for the ancient Romans - so the additional day was at the end of the y

This would be a lot easier if the year ran from March 1 to February 28/29. Which it did, for the ancient Romans - so the additional day was at the end of the year. This also explains why September through December come from roots meaning 7 through 10, despite being the ninth through twelfth months.