Almost every four years it’s a leap year. Leap years have one extra day due to an astronomical fact about the earth’s revolution around the sun. It takes roughly 365 and one quarter days to go around the sun, however the standard calendar year lasts only 365 days. Were we to ignore the extra quarter of a day, strange things would eventually happen to our seasons - like winter and snow in July. To counteract the accumulation of additional quarters of a day, theGregorian calendar adds an extra day of February 29 nearly every four years.
Until Julius Caesar came to power, people observed a 355-day calendar - with an extra 22-day month every two years. But it was a convoluted solution to the problem and feast days began sliding into different seasons. So Caesar ordered his astronomer to simplify things. Astronomer opted for the 365-day year with an extra day every four years to scoop up the extra hours. This is how the 29 February was born.
Why is February 29, not February 31, a leap day? All the other months have 30 or 31 days, but February suffered from the ego of Roman Emperor Caesar Augustus. Under Julius Caesar, February had 30 days, but when Caesar Augustus was emperor he was peeved that his month - August - had only 29 days, whereas the month named after his predecessor Julius - July - had 31. He pinched a couple of days for August to make it the same as July. And it was poor old February that lost out.
Text source: Compilation from widely disseminated descriptions in online news sources.