Today, 29 February (sometimes known as St Oswald’s Day), is a date for which people born on that day (sometimes called "leaplings") get to celebrate their birthday, at least one the correct day, not one day earlier or one day later.
But why have leap years, and have a 29 February only once every four years? Actually, it's less than once every four years as one does not count centuries, but one does count every fourth century (i.e. every 400 years), but that is a moot point. It is to do with the duration of the earth's orbit around the sun and, without have an extra day (almost) once every four years, it is to keep the months and seasons in sync with annual events, including equinoxes and solstices. It takes the Earth 365.242190 days to orbit the Sun, or 365 days 5 hours 48 minutes and 56 seconds; the extra 5 hours 48 minutes and 56 seconds needs to be accounted for somehow.
Historically, Julius Caesar had a "Julian" calendar that included leap years every three year; his successor, Augustus, changed it, and so did Pope Gregory who published the Gregorian calendar in 1582.
For hundreds of years, different traditions have sprung up in different cultures around leap year and, specifically, 29 February.
In some countries, including Ireland, it is traditionally the day (known as "Bachelor's Day" or "Ladies Privilege") on which women ask men to marry them as traditionally it has been the other way round.
And Luxembourg has some such Leap Year traditions too, with Bretzelsonndeg (Bretzel Sunday, which falls on 10 March this year) encouraging women and men to swap traditional roles, with women this year given their men a pretzel, another role reversal.
In Scotland, it used to be considered unlucky for someone to be born on leap day, just as Friday 13th is considered an unlucky day by many. Greeks consider it unlucky for couples to marry during a leap year, and especially on Leap Day.
In France, La Bougie du Sapeur, the world's only "quarrennial" newspaper - that is published just once every four years - hits the newsstands; the 20-page tabloid-format newspaper started life back in 1980 and has a print run of 200,000 copies.
