adv. phr. Very rarely; very seldom; almost never. Coin collecting is interesting, hut you find a valuable coin only once in a blue moon. Once in a blue moon someone grows a very pale marigold, but no truly white marigolds have been raised.
ONCE IN A BLUE MOON
Meaning of ONCE IN A BLUE MOON in English
American idioms English vocabulary. Английский словарь американских идиом. 2012