A common year is 365 days long, but a leap year adds an extra day to the end of February every four years making it 366 days long. Why do we do this?
Our year is based on how long it takes Earth to orbit the sun. The earth makes one complete revolution around the sun in 365.242 days, so we include February 29 to make up for the lost six hours every year. If we didn’t, our calendar would be off by almost a month after just 100 years!