A full moon occurs on average every 29.53 days. For every month except for February, the time between full moons is less than a calendar month.
As a result, there are times when there are two full moons in a month. When this occurs, the second full moon of the month is called a "Blue Moon." This typically occurs about every 33 months. A "Harvest Moon" is the term given to the full moon that occurs closest to the autumnal equinox. Each month's full moon is also known by a particular name. For example, March's moon is known as the "Sap Moon."