This is used in order to say that something rarely or almost never happens.


I like skating, but I only get a chance to go once in a blue moon.

Eliot never had much idea of what it means to be a father, and since the divorce he just comes round to see the boys once in a blue moon.


Source: Longman Idioms Dictionary

Meaning

A very rare event. According to popular usage, a blue moon refers to the second full moon that occurs in any calendar month. On average, there will be 41 months that have two full moons in every century. By that calculation 'once in a blue moon' means once every two-and-a-half years.

Origin

There are rare examples of the moon actually appearing blue, after volcanic eruptions or unusual weather conditions. That's not what is being referred to in this phrase though. Just the opposite in fact as a blue moon was originally cited as something that was absurd and therefore impossible; only later did it come to mean unlikely.

Since 1819, The Maine Farmers' Almanac has listed the dates of forthcoming blue moons. The compilers of the almanac didn't use the 'second full moon in a month' definition that is now generally accepted but instead defined a blue moon as the third full moon in a season which has four.

The 'blue moon' expression itself is old and dates back to mediaeval England. For example, a work by William Barlow, the Bishop of Chichester, the Treatyse of the Buryall of the Masse, 1528, which is more commonly known by its first line, Rede me and be nott wrothe, For I say no things but trothe included a reference to a blue moon:

"Yf they saye the mone is belewe, We must beleve that it is true."

A blue moon was a synonym for absurdity - considered to be as likely as the moon being made out of green cheese. This imagery was called on in John Frith's exhaustively entitled essay A pistle to the christen reader; the reuelation of antichrist: antithesis wherein are compared togeder Christes actes and oure holye father the Popes, 1529

"They wold make men beleue... that ye mone is made of grene chese."


http://www.phrases.org.uk/meanings/269500.html