A full moon occurs when the Moon is fully illuminated by the Sun, and this occurs when the Moon is exactly opposite the Sun in the sky. Since the Moon orbits the Earth every 27.3 days, and the Earth orbits the Sun every 365 days, there will be times when there are two full moons in a calendar month.

A full moon occurs every 29.53 days, which is the average time between full moons. Since there are 30 or 31 days in most months, it is possible for there to be two full moons in a single calendar month. This occurs because the first full moon of the month can occur at the beginning of the month, and the second full moon can occur near the end of the month.