The Earth revolves around the Sun once every year, which we commonly refer to as a "year" or an orbital period. This orbital period, also known as the Earth's revolution, takes approximately 365.25 days to complete. To account for the extra fraction of a day, we add an extra day to the calendar every four years, known as a leap year. During a leap year, February has 29 days instead of the usual 28. This adjustment helps to keep our calendars aligned with the Earth's orbit around the Sun.