Rare occurrence

Wikipedia Word definitions in Wikipedia
" Blue Moon " is a classic popular song written by Richard Rodgers and Lorenz Hart in 1934 , and has become a standard ballad . It may be the first instance of the familiar " 50s progression " in a popular song. The song was a hit twice in 1949 with successful ...

WordNet Word definitions in WordNet
n. a long time; "something that happens once in blue moon almost never happens"

Wiktionary Word definitions in Wiktionary
n. 1 (context rare English) The moon tinted towards blue as it appears in the sky, caused by dust or smoke in the atmosphere. 2 An extra full moon that appears in a subdivision of a year, namely: 3 # The third full moon in a quarter that contains four rather ...

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
1821 as a specific term in the sense "very rarely," perhaps suggesting something that, in fact, never happens (compare at the Greek calends , and the native in the reign of Queen Dick and Saint Geoffrey's Day "Never, there being no saint of that name," ...