Blue Moon

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 (on average about every 2.7154 years), there is an extra full moon. The extra moon is called a "Blue Moon". Different definitions place the "extra" moon at different times.