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 calendar year contains an excess of roughly eleven days. 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 Farmer's 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 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 saying "once in a blue moon."

Early English and Christian usage

The earliest recorded English usage of the term "blue moon" was in 1528 in a pamphlet violently attacking the English clergy, entitled Rede Me and Be Not Wrothe me and be not angry: "Yf they say the mone is belewe / We must believe that it is true" they say the moon is blue, we must believe that it is true.

Some interpret this "blue moon" as relating to absurdities and impossibilities, and a similar moon-related adage was first recorded in the following year: "They would make men beleue ... that þe Moone is made of grene chese" would make men believe ... that the moon is made of green cheese.

An alternative interpretation uses the other old English meaning of "belewe" (which can mean "blue" or "betrayer"). The church was responsible for the calendar and used the complex computus to calculate the important date of Easter, which is based on the full moon. Lent falls before Easter starting at the beginning of the Lent moon cycle (late winter moon). The next moon is the egg moon (early spring moon), and Easter usually falls on the first Sunday after the full egg moon. Every one to three years the Lent and egg moons would come too early, so the clergy would have to tell people whether the moon was the Lent moon or a false one, which they may have called a "betrayer moon".