Rare occurrence

Wikipedia Word definitions in Wikipedia
Blue Moon is an Austrian movie written and directed by Andrea Maria Dusl in 2002 . The road movie romantic comedy is Dusl's first as a director.

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," ...