A full moon occurs every 29.5 days, which is the length of time it takes to complete one lunar phase cycle. The lunar phases occur as the moon orbits the earth.
The portion of the moon that is seen from earth is the part that is illuminated by the sun. The lunar cycle runs from new moon, or a totally dark moon, to the next new moon.
When it is full moon, the moon is positioned behind the earth with regard to the sun. As the moon rises it is fully illuminated by the sun.
During the new moon phase, the moon is located between the earth and the sun, so none of the side of the moon that is facing the earth is illuminated.