A blue moon is a full moon that is not timed to the regular monthly pattern. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles, each solar calendar year contains an excess of roughly eleven days compared to the lunar year. The extra days accumulate, so that every two or three years (on average about every 2.7154 years), there is an extra full moon. The extra moon is called a "blue moon." Different definitions place the "extra" moon at different times.
• 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 which came too early had no folk name – and was called a blue moon – bringing 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.
The term "blue moon" is commonly used metaphorically to describe the rarity of an event, as in the idiomatic expression, "once in a blue moon."