_ The Farmers' Almanac defines a blue moon as an extra
full moon that occurs 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. Most years have twelve full moons that occur
approximately monthly. However, since the lunar cycle has less days than
the solar calendar year, every two or three years those days accumulate
and result in 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.
Next Blue Moon: July 31, 2015 Blue Moons After: January 31, 2018 March 31, 2018 October 31, 2020 (dates based on EST)
In calculating the dates for Lent and Easter, the Clergy identify the Lent Moon. It is thought that historically when the moon's timing was too early, they named an earlier moon as a "betrayer moon" (belewe moon), thus the Lent moon came at its expected time.
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.
Recent popular usage defined a blue moon as the second full moon in a calendar month, stemming from an interpretation error made in 1946 that was discovered in 1999. For example, December 31, 2009 was a blue moon according to this usage.
A "blue moon" is also used colloquially to mean "a rare event", reflected in the phrase "once in a blue moon".