Folklore gave each moon a name according to its time of year. A moon that came too early had no folk name, and was called a blue moon, retaining the correct seasonal timings for future moons.
The Farmers' Almanac defined blue moon as an extra full moon that occurred in a season; one season was normally three full moons. If a season had four full moons, then the third full moon was named a blue moon.
A "blue moon" is also used colloquially to mean "a rare event", reflected in the phrase "once in a blue moon".
A blue moon can refer to the third full moon in a season with four full moons. Most years have twelve full moons that occur approximately monthly. In addition to those twelve full lunar cycles, each solar calendar year contains roughly eleven days more than the lunar year of 12 lunations. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. Lunisolar calendars have rules about when to insert such an intercalary or embolismic ("leap") month, and what name it is given; e.g. in the Hebrew calendar the month Adar is duplicated. The term "blue moon" comes from folklore. Different traditions and conventions place the extra "blue" full moon at different times in the year. In the Hindu calender, this extra month is called 'Adhik(extra) masa (month)'. It is also known as purushottam maas, so as to give it a devotional name.
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