On December 18, 2021 the moon reaches its maximum brightness. Therefore it is called full moon. 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 longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun. As seen from Earth, the hemisphere of the Moon that is facing the earth 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, completely unilluminated.
The time interval between similar lunar phases is on average about 29.53 days. Therefore, in those lunar calendars in which each month begins on the new moon, the full moon falls on either the 14th or 15th of the lunar month. Because lunar months have a whole number of days, lunar months may be either 29 or 30 days long.
Full Moons are traditionally associated with temporal insomnia, insanity and various "magical phenomena" such as lycanthropy. Psychologists, however, have found that there is no strong evidence for effects on human behavior around the time of a full moon. They find that studies are generally not consistent, with some showing a positive effect and others showing a negative effect. In one instance, the British Medical Journal published two studies on dog bite admission to hospitals in England and Australia. The study of the Bradford Royal Infirmary found that dog bites were twice as common during a full moon, whereas the study conducted by the public hospitals in Australia found that they were less likely. Full moons trigger deer movement in North America. Hunters and fisherman rely heavily on moonphases. (Source: Wikipedia, license: GNU FDL)