A blue moon is an additional full moon that appears in a subdivision of a year, either the third of four full moons in a season or, a second full moon in a month of the common calendar. The phrase has nothing to do with the actual color of the moon, although a literal “blue moon” (the moon appearing with a tinge of blue) may occur in certain atmospheric condition.
Owing to the rarity of a blue moon, the term “blue moon” is used colloquially to mean a rare event, as in the phrase “once in a blue moon”.
