|1.||the second full moon occurring within a calendar month|
|2.||informal once in a blue moon very rarely; almost never|
Yf they say the mone is blewe,Though this might refer to calendrical calculations by the Church, so that the general sense of the term and the specific one (commonly misinterpreted as "second full moon in a calendar month," but actually a quarterly calculation) are difficult to disentangle. In either case, the sense of blue is obscure. Literal blue moons do occasionally occur under extreme atmospheric conditions.
We must beleve that it is true.