<P> if (year is not divisible by 4) then (it is a common year) else if (year is not divisible by 100) then (it is a leap year) else if (year is not divisible by 400) then (it is a common year) else (it is a leap year) </P> <P> February 29 is a date that usually occurs every four years, and is called leap day . This day is added to the calendar in leap years as a corrective measure, because the Earth does not orbit the sun in precisely 365 days . </P> <P> The Gregorian calendar is a modification of the Julian calendar first used by the Romans . The Roman calendar originated as a lunisolar calendar and named many of its days after the syzygies of the moon: the new moon (Kalendae or calends, hence "calendar") and the full moon (Idus or ides). The Nonae or nones was not the first quarter moon but was exactly one nundina or Roman market week of nine days before the ides, inclusively counting the ides as the first of those nine days . This is what we would call a period of eight days . In 1825, Ideler believed that the lunisolar calendar was abandoned about 450 BC by the decemvirs, who implemented the Roman Republican calendar, used until 46 BC . The days of these calendars were counted down (inclusively) to the next named day, so February 24 was ante diem sextum Kalendas Martias ("the sixth day before the calends of March") often abbreviated a.d. VI Kal . Mart . The Romans counted days inclusively in their calendars, so this was actually the fifth day before March 1 when counted in the modern exclusive manner (not including the starting day). </P> <P> The Republican calendar's intercalary month was inserted on the first or second day after the Terminalia (a.d. VII Kal . Mar., February 23). The remaining days of Februarius were dropped . This intercalary month, named Intercalaris or Mercedonius, contained 27 days . The religious festivals that were normally celebrated in the last five days of February were moved to the last five days of Intercalaris . Because only 22 or 23 days were effectively added, not a full lunation, the calends and ides of the Roman Republican calendar were no longer associated with the new moon and full moon . </P>

When do we not have a leap year