There is a common expression in the English language (refer) to a blue moon. When people say something happens" only once in a blue moon", they actually intend to indicate the (frequent) of it. 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.
The expression "a blue moon" has come to refer to second full moon occurring in any given calendar month. A second full moon is called a blue moon not because it is (particular) blue or is any different in hue (色彩) the first full moon of the month. Instead, it is called a blue moon because it is so rare. The moon needs a little more than 29 days (complete) the cycle from full moon to full moon. Because every month except February has more than 29 days, every month has at least one full moon, except February, which has a full moon there is a full moon at the end of January and another full moon at the end of March. It is on the occasion a given calendar month has a second full moon that a blue moon occurs. It happens only three or four (time) in a decade.
