(1) Another common expression in the English language refers to a blue moon.When people say that something happens "only once in a blue moon," they mean that it happens only very rarely, once in a great while. This expression has been around for at least a century and a half, there are references to this expression that date from the second half of the nineteenth century. (2) The story of this expression is so strange. The expression "a blue moon" has come to refer to the second full moon occurring in any given calendar month.Because every month except February has more than 29 days, every month will have at least one full moon (except February, which will have a full moon unless there is a full moon at the very end of January and another full moon at the very beginning of March).It is on the occasion where a given calendar month has a second full moon that a blue moon occurs.A second full moon is not called a blue moon because it is particularly blue or is any different in hue from the first full moon of the month.The moon needs a little than 29 days to complete the cycle from full moon to full moon.This does not happen very often, only three of four times in a decade.Instead, it is called a blue moon because it is so rare.