What is a leap year? What is a common year?
A leap year contains one extra day, February the 29th, over an ordinary year.
In other words, a leap year is 366 days long, the ordinary year is one day shorter: 365 days.
A leap year is a calendar year that contains 366 days as compared to the 365 days contained by an ordinary calendar year. A year that is not a leap year is called a common year.
A leap year contains one additional day over the common year: the February 29th (the leap day). In a common year the month of February has only 28 days and February 29th does not exist.