<P> When it was thought necessary to add (every two years) an intercalary month of 22 or 23 days, so that the civil year should correspond to the natural (solar) year, this intercalation was in preference made in February, between Terminalia (23rd) and Regifugium (24th). </P> <P> The set leap day was introduced in Rome as a part of the Julian reform in the 1st century BC . As before, the intercalation was made after February 23 . The day following the Terminalia (February 23) was doubled, forming the "bis sextum"--literally' twice sixth', since February 24 was' the sixth day before the Kalends of March' using Roman inclusive counting (March 1 was the Kalends of March and was also the first day of the calendar year). Inclusive counting initially caused the Roman priests to add the extra day every three years instead of four; Augustus was compelled to omit leap years for a few decades to return the calendar to its proper position . Although there were exceptions, the first day of the bis sextum (February 24) was usually regarded as the intercalated or "bissextile" day since the 3rd century AD . 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, although this has only been formally enacted in Sweden and Finland . In Britain, the extra day added to leap years remains notionally the 24th, although the 29th remains more visible on the calendar . </P> <P> A person born on February 29 may be called a "leapling", a "leaper", or a "leap - year baby". In non-leap years, some leaplings celebrate their birthday on either February 28 or March 1, while others only observe birthdays on the authentic intercalary date, February 29 . </P> <P> The effective legal date of a leapling's birthday in non-leap years varies between jurisdictions . </P>

When do leap year babies celebrate their birthdays