Calendar units are the basic divisions of time used to organize days, weeks, months, and years. The most common calendar unit is the day, which is based on the Earth's rotation on its axis. A week consists of seven days, while a month typically has 28 to 31 days, depending on the specific month. The year is another important calendar unit, representing the time it takes for the Earth to orbit the Sun, which is about 365.25 days. To account for this extra quarter day, a leap year occurs every four years, adding an extra day to the month of February.