"The Gregorian calendar repeats itself every 400 years, which is exactly 20,871 weeks including 97 leap days. Over this period, February 29 falls 13 times on a Sunday, Tuesday, or Thursday; 14 times on a Friday or Saturday; and 15 times on a Monday or Wednesday.
The concepts of the leap year and leap day are distinct from the leap second, which results from changes in the Earth's rotational speed.
Adding a leap day (after 23 February) shifts the commemorations in 1962 Roman Missal.
The leap day was introduced as part of the Julian reform. The day following the Terminalia (February 23) was doubled, forming the "bis sextum"—literally 'double sixth', since February 24 was 'the sixth day before the Kalends of March' using Roman inclusive counting (March 1 was the 'first day'). Although exceptions exist, the first day of the bis sextum (February 24) was usually regarded as the intercalated or "bissextile" day since the third century.[1] February 29 came to be regarded as the leap day when the Roman system of numbering days was replaced by sequential numbering in the late Middle Ages."
Wikipedia.