What exactly is a blue moon?

A blue moon can refer to the third full moon in a season with four full moons, or the second full moon of a calendar month. 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 calenders have rules on when to insert such an intercalary of embolismic ("leap") month, and what name it is given; e.g. in the Hebrew calender, the month Adar is duplicated. The term "blue moon" comes from folk lore. Different traditions and conventions place the extra "blue" full moon at different times in the year.
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.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.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". Last year on new years when we had a blue moon i really couldn't tell much difference.
The most literal meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by the Tyndall effect(in turn, caused by smoke or dust particles in the atmosphere), as has happened after forest fires in Sweden and Canada in 1950 and 1951. As well as after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years. Other less potent volcano's have also turned the moon blue. People saw blue moons in 1983 after the eruption of the El Chichon volcano in Mexico, and there are reports of blue moons caused by Mt.St.Helen in 1980 and Mount Pinatubo in 1991.Using the Farmers' Almanac definition of blue moon (meaning the third full moon in a season of four full moons), blue moons occur:
* November 21, 2010
* August 21, 2013
* May 21, 2016