Bissextile

Leap day is not the 29th day of February. It's the 24th. Rather, two days are counted as day 24. The Romans called the first day of the month Kalends, which is where we get the word "calendar." They counted down the days before Kalends, so the last day of February, the day before the Kalends of March, was numbered II. The 24th day was numbered VI. In leap years, there was an extra day VI in February, which was called bissextile. (Not bisexual!)

So those born on the last day of February in a leap year would cerebrate the last day every year, since it was counted as the 28th day, not 29. Likewise, those born on either the first or second 24 would celebrate on the 24th every year. No waiting four years!