Though the term is often used metaphorically to describe a rare event, a blue moon is actually, in astronomical terms, the second full moon within a calendar month or the third full moon in a three-month calendrical season that has four full moons. While most years have 12 full moons that occur approximately once a month, each solar year has about 11 days more than the lunar year. These extra days accumulate, and, every few years, an “extra” full moon appears. When is the next blue moon? Discuss
Source: The Free Dictionary