February 24 is the old Lead Day as opposed to February 29 as it is celebrated today. The Roman calendar was based on the phases of the moon. Kalends was what we would refer to as the first day of the month and it was the new moon, or when there was no moon visible in the sky. Nones was the first quarter moon, which would be around the 5th to the 7th day of the month. And Ides would be the full moon, corresponding to the 13th-15th day of the month.
To reconcile the lunar calendar to the solar year, the sixth day before the Kalends of March was simply said to last for 48 hours. When the extra hours finally began to be reckoned as two separate days, they still took place after February 23. February 29 has been popularly understood as the leap day of leap years since the beginning of sequential reckoning of the days of months in the late Middle Ages.