It is Leap Year Day. In Latin this is bisextilis.
For this intercalary day in the Roman Martyology we find this:
When there isn’t a 29 February, the saints are observed on 28 February.
In 46 BC, on the advice of the Alexandrian astronomer Sosigenes, G. Iulius Caesar created a calendar system that added one day every four years to make up for the fact that the Earth’s year is slightly more than 365 days. The Earth circle the Sun in slightly more time than it takes for the Earth to rotate 365 times (365.24219). Calendar years with 365 drift from the actual year by about 1 day every 4 years. After a while the month named after Caesar, July occur during the winter (in the Northern hemisphere).
Caesar’s Julian Calendar was maintained until 1582 when Pope Gregory XIII determined that in his Gregorian Calendar leap days would not occur in years ending in OO, unless the year is divisible by 400.