Which is the leap day in the leap year? Many would answer "February 29th". That answer is not correct. As surprising as it could be, the answer is "February 24th". How do we get this answer? The explanation is as follows:
The old Roman Calendar was a lunar calendar. After the addition of February and January, it had a length of 355 days. Every other year an extra month had to be added in order for the calendar to keep pace with the seasons. This month was called Mercedinus, later Intercalaris, and had a duration of 23 or 24 days (or other lengths also, as decided by the Pontifex Maximus). This "leap month" was added after February 23rd. The rest of the February days were placed after Mercedinus ended. This is how the tradition of adding leap days to adjust the calendars to the solar cycle started. Today, we repeat February 24th twice in order to make February a 29 day month every 4 years.