A leap year contains 366 days. This additional day occurs every four years to help synchronize the calendar year with the solar year, which is the time it takes for the Earth to orbit the sun—approximately 365.25 days. Including an extra day, February 29, in the leap year accounts for this discrepancy. However, there are exceptions to the rule, such as years divisible by 100 not being leap years unless they are also divisible by 400. This system ensures the calendar remains accurate over long periods.