From wikipedia (https://en.wikipedia.org/wiki/Blue_moon) : A blue moon is an additional full moon that appears in a subdivision of a year: either the third of four full moons in a season, or a second full moon in a month of the common calendar. Moreover it is a song :-)

The phrase basically means "once in a long while"

"Once in a blue moon" is an idiom meaning "very rarely." Nobody has ever come up with a convincing explanation of its origin--particularly, nobody knows what is supposed to be "blue." The idea that it has something to do with two full moons in one month is, forgive me, nonsense. I'm surprised the Wikipedia article doesn't say a bit more about this. I never heard any such thing before the 1980s, and it's been traced to one particular program on NPR. NPR itself says: http://www.npr.org/sections/thetwo-way/2015/07/31/428012555/tonight-look-for-a-rare-but-not-quite-blue-moon "a secondary definition — the one about two full moons in a month — popped up (as an error, it turned out) in a 1946 issue of Sky & Telescope. Years later, in 1980, the mistake was amplified by the public radio program StarDate, and later, by the board game Trivial Pursuit. It stuck." Or perhaps it's not nonsense because it has now been repeated so often that it has become true.

Ah I can explain this in Cantonese, but unfortunately I can't write it. Its like the saying in Cantonese that directly translates to "It doesn't happen in 9 autumns." It means the same thing as once in a blue moon where something rarely happens just like a blue moon or 9 autumns.

Kinson, "once in a blue moon" means very seldom. Do you often go to the theatre? No, very seldom, once in a blue moon.