A blue moon is an additional full moon that appears in a subdivision of a year: the third of four full moons in a season. 'Once in a blue moon' is a common expression that has been used for a long time, and which means 'not very often,' or 'very rarely.' It often refers to an extra full moon; however, it has been used to describe the way the moon actually looked, when for different reasons it had turned a blueish colour. [UK Boat #016, 2017-06-29]