Why do we have leap years? It's because it takes the Earth exactly 365.24 days to orbit the sun. This means a "leap day" - February 29th - must be added every four years to keep us in sync with the cosmic clock. Here are some other things worth knowing about leap years...
The odds of being a leapling
At 1 in 1,461, the chances of being born on a Leap Day are perhaps higher than you think. Currently, there are estimated to be around 4 to 5 million leaplings in the world. Famous leaplings from the past include the great composer Rossini and English poet John Byrom. But James Milne Wilson, the one-time Premier of Tasmania during the Victorian era, managed a Leap double. Not only was he born on a Leap Day - 29th February 1812 - but he also died on the 29th February in 1880. Counting only his Leap Day birthdays, he lived to the ripe old age of 17.
Leap Years have to be divisible by 400
Everyone knows that Leap Days occur every four years. Except they don't... quite. Because the Earth's orbit of the sun is not very mathematically neat when it comes to organising time in a way that humans can make sense of it, we have a leap year every four years, but only if that year is divisible by both 100 and 400. If it is only evenly divisible by 100, then it's just a normal year, with no Leap Day. So 2016 is a Leap Year because it's divisible by 4, 100 and 400. The year 1900 wasn't a Leap Year, because it's not divisible by 400. This mathematical headache is probably not something most of us need to worry about though, since the next "missed" leap year won't be until 2100.
The Romans leapt months
If leap days seem to mess around with time, imagine what it was like before the introduction of the Gregorian Calendar. In Julius Caesar's day, the calendar worked on a 355-day year, with an extra 22-day "leap month" added every two years. Caesar had it simplified to the 365-day system we still use, and gave his name to the month we now call July. By the way, there's a long-standing legend that Caesar's successor, Augustus, is the reason that February is such a short month, because he "stole" a day from February to lengthen the month named after him - August. But this popular story has no basis in historical fact.
Say yes or pay a fine
The tradition of women proposing on leap days has been attributed to St Brigit of Kildare, who apparently complained to St Patrick that women had to wait far too long for their menfolk to propose. According to this legend, St Patrick duly allowed women to make the first move on leap days only. Another story has "Queen Margaret of Scotland" decreeing in 1288 that men refusing a leap year proposal would be liable for a fine. However, this is now widely believed to be a historical hoax. What does seem to be true is that mythology throughout Europe has stories about women proposing to men on the 29th February and men having to pay fines in the form of gloves and skirt fabric if they refuse.
Sweden once had February 30th
In 1712, February 30th popped into brief existence in Sweden. Twelve years earlier, in 1700, Sweden had decided to transition to the Gregorian calendar from the Julian one. It meant that they had some extra days to get rid of in order to become synchronized, which they decided to do gradually, skipping just the leap days. However, the Great Northern War broke out in 1700, and proved to be a lengthy, violent struggle, and the Swedes understandably forgot to skip the next two leap days of that decade. By 1712, the Swedes decided it would be easier to revert back to the Julian calendar, by adding an extra leap day on top of the normal leap day of the year. Hence, February 30th came into existence for that year only. It took until 1753 for the transition to the Gregorian system to finally occur, but February 30th was never seen again.