A blue moon is a full moon that is not timed to the regular monthly pattern. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles, each solar calendar year contains an excess of roughly eleven days compared to the lunar year. The extra days accumulate, so that every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. The extra moon is called a "blue moon." Different definitions place the "extra" moon at different times. Historically, moons were given folk names, twelve each year, to help people to prepare for different times of the year and the related weather and crop needs. Names varied with locality and culture, often with descriptive names such as harvest moon, growing moon, snow moon, and egg moon. Most years have 12 moons (giving 12 names), but in the years with thirteen full moons the monthly "seasons" would be expected to come too early – for example, hens would not recommence laying their eggs by the fourth full moon since it was still too cold – so the early moon was named a "blue moon". This then re-aligned the rest of the year’s moons and "seasons".
The origin of the term "blue moon" is steeped in folklore, and its meaning has changed and acquired new nuances over time. Some folktales say that when there is a full blue moon, the moon had a face and talked to those in its light. According to Rainer Werner Fassbinder, "Every seventh year is one of the moon. In such years people suffer increasingly from emotional ups and downs and depression. In 2009 the 13th moon of the year happens on December 31 and is combined with a partial lunar eclipse in some parts of the world including asia, Australasia, Europe and Africa. The moon will be found in the constellation Gemini on December 31, 2009.