What is a Blue Moon?

A "blue moon" is the second full moon in a month, which happens every two to three years. The use of the term "blue moon," meaning infrequently reoccurring, dates to 1821.

Source: 

Post-Crescent, 7-30-1985; Oxford English Dictionary

Notes: 

See the Sky and Telescope article What's a Blue Moon? or the Blue Moon web site for more information.

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