Full moon is a lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent (ecliptic) longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.At this time, as seen by viewers on Earth, the hemisphere of the Moon that is facing the earth (the near side) is almost fully illuminated by the Sun and appears round. Only during a full moon is the opposite hemisphere of the Moon, which is not visible from Earth (the far side), completely unilluminated.
The average lunar month is about 27.3 days long, so that the full moon falls on either the 14th or 15th of the lunar month in those calendars that start the month on the new moon. In any event, as lunar months are counted in discrete numbers of days, lunar months are said to be either 27 or 28 days long.