once in a blue moon

once in a blue moon  {adv. phr.}
Очень редко; практически никогда.
Coin collecting is interesting, but 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.
Ср.
Категории: {adv. phr.}

'once in a blue moon' on video