Why a Leap Year?
In 2016 we add February 29 to the calendar. Leap years, which debuted in the Gregorian calendar in the 16th Century, occur every four years, with exceptions, to keep our human time-keeping devices in sync with the natural rhythm of the earth.
Here's my simple video explanation at http://youtu.be/e10PwkScF-I.
[Note: In the opening sentences I I muddy the water by suggesting we've been taught "earth rotates 365 days as it goes around the sun." For clarity, the earth rotates about 365 times (i.e., 365 days) as it revolves around the sun once (i.e., one year. ]
Analogy: Track Practice
A perspective different from the video above is to imagine you are a runner at a track practice. The coach sets a pace for you to sprint around the track. "Go!", she says, and you're off. However, one stride before you get back to the starting point she says "Go!" again. You're behind, but you start the next split. Maintaining your pace, when you approach the starting point the second time you're now two paces behind when the coach shouts "Go!" After the third lap, three strides behind. After the fourth lap, four strides behind.
This time, with you four strides behind, the coach allows you to catch up. She waits until you've reached the starting line before barking "Go!" again.
As a runner, you're moving straight ahead, but as a planet you'd have to rotate as you revolve around the track--a quarter turn for each stride. At the start, you (the running rotating planet) are facing forward. After the first lap (when you're short of the finish and when coach says "Go!") you finish facing left a quarter turn, or the inside of the track. After the second split, you're facing backward. After the third split, you're facing right, or the outside of the track. After the fourth split, you're facing forward again. When the coach allows you four strides to get back to the starting line, with each stride you rotate the quarter turn, so after four strides you're back at the starting line and facing straight ahead again. With each stride equivalent to 6 hours, those four quarter-turn strides are your leap day.