2016 is a leap year, and February, the shortest month of the year, gains an extra day—February 29^{th}, otherwise known as leap day!

**So why do we have leap years and days?** Leap years and days are adjustments built into the calendar’s structure to keep the civil calendar in sync with the solar year—and therefore, the seasons! First established by Julius Caesar when he implemented the Julian calendar in 46 BC, the concept of leap years and days carried over when the Gregorian calendar (today’s established civil calendar) was introduced in 1562. While most calendar years have 365 days, the actual solar year is approximately 365 days *and 6 hours* long! This means that every non-leap year, our calendar is off by about a quarter of a day—leap days are thrown in to bring the calendar back into alignment!

Though conventional wisdom holds that leap years occur every four years, the truth involves a little more math, because the solar year is only *roughly* 365.25 days! These little inaccuracies add up. So, we say that leap years *generally* occur every four years… *unless the year is divisible by 100 but not by 400*.

This video (courtesy of slate.com) explains it beautifully:

Those lucky enough to have birthdays on February 29 are affectionately called “leaplings”! It’s said that the odds of being born on a leap day are about 1 in 1,461. This is calculated based on the idea that leap years happen every four years: three of these years have 365 days in them, with the fourth year, a leap year, having 366 days. The sum of all of these days is 365+365+365+366 = 1461. Only one of these days is a leap day, which gives us odds of 1 in 1,461. Based on the slight inaccuracies in leap year calculation outlined above (as well as variations in human births based on location and time of year—more babies are born during certain times of year than others and this varies from place to place), can you see why this calculation may also be a little off?