So the simple answer is that we put the leap day at the end of February because the Romans did. On a leap day, they added a second sixth kalends of March, which they called the “bissextile day”, that is the second sixth day. In older writings of various kinds, you will still see people call the leap day, February 29, the bissextile day. This practice of adding a leap day in February continued into the Middle Ages and was taught in monastic classrooms. At any point, the leap day could have been changed to something that made more sense in a modern calendar.

Source: Irish Examiner February 29, 2024 01:46 UTC