Rare occurrence

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," ...

Wikipedia Word definitions in Wikipedia
Blue Moon is the debut studio album by the doo-wop group The Marcels . It was released in 1961 on Colpix Records and included 12 songs. The album was available in mono , catalogue number CP-416. Blue Moon was produced and arranged by Stu Phillips and was ...

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 ...