29.9

Blue moon is the appearance of the third full moon in a season with four full moons. It is never really blue. The term has also been used to refer to the second full moon in a month. The average lunar cycle is 29.530589 days, there are 365 days in a solar year, 365 divided by 29.530589 days equals about 12.36 lunar cycles in a solar year. In the gregorian calendric cycle, there are 12 months (the word month is derived from moon) In addition to those twelve months representing 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 calendar, this extra month is called 'Adhik (extra) masa (month)'. It is also known as purushottam maas, so as to give it a devotional name. Info via wikipedia