In modern vernacular, the term "blue moon" most often refers to the occasion of an extra full moon occurring in a month. Most months have only one full moon, but sometimes there will be a second full moon in a month known as the blue moon. This can occur during any month of the year, except February.

Upcoming dates for the blue moon according to this definition:

2015: July 2, July 31
2018: January 2, January 31
2018: March 2, March 31
2020: October 1, October 31

In 1946, Sky & Telescope magazine erroneously described a blue moon as an extra full moon in a month and this definition caught on leading to the modern usage.

Prior to the gaffe of 1946, the blue moon always referred to an extra full moon in a season, that is, the span of time between the equinoxes and the solstices. Most often there are three full moons during this period, but occasionally there is the fourth. Since each full moon has a name associated with the season, an additional moon can cause the names to line up improperly so the additional moon needs a different name and Blue Moon works. It is the name given to the third moon in a season with four full moons.

For the Christian church, an extra moon before the Lenten moon causes it to not fall during the Lenten season, so this moon was called the betrayer moon, Old English belewe.

This happens about once every 2.7 years.

Some upcoming dates for the blue moon according to this definition:

May 21, 2016
May 18, 2019
August 22, 2021
August 19, 2024

A blue moon by the first definition happens much more often then the second definition and so when a blue moon is discussed, it most often refers to the first definition. However, the first definition depends on arbitrarily imposed month lengths while the second depends on natural cycles. For most Pagans, witches, astrologers and astronomers, the second definition based on the seasons is correct and the most useful. However, it is also much more difficult to keep track of and so the first definition is enough for many people.

Used this way, the term does not have anything to do with the moon's color. The moon can appear blue when volcanic eruptions or large fires kick particles into the atmosphere, but this is an unrelated phenomenon.

The phrase "blue moon" may older than either of the above uses. It has been used since at least the 16th century to say something is ridiculous or unlikely. For example one might say "He is so contrary, he would argue that the moon is blue" or someone might say "I will wear that silly hat when the moon is blue". This is a case when the phrase existed long before its definition.

The Blue Moon and Magick

Some witches consider the blue moon to be twice as powerful or at least more powerful than the full moon it precedes and so consider it a good time to perform any spells that need an extra kick.

In common metaphor, "Once in a blue moon" refers to something that happens very rarely or has a low statistical chance of happening, but is not impossible. This symbolism can be incorporated into magick aimed and causing something to happen that would only happen "once in a blue moon".

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