“Once in a blue moon” is an expression referring to an event that does not happen very often. A blue moon happens about once every 2.7 years when the Moon is full twice during the same calendar month. The exact origin of the phrase is not certain, but it seems to be about 400 years old and comes from English roots. To say that the Moon was blue would be to express that something was absurd or unlikely. Thus, the phrase did not mean that the Moon was literally blue.

Sometimes, however, the Moon has been known to take on a bluish hue, especially when there is a lot of ash or dust in the atmosphere, such as after a large forest fire or volcanic eruption.