Very rarely; very seldom; almost never.
Очень редко, почти никогда.
Once in a blue moon my wife and I eat at a very expensive restaurant.
Once in a blue moon someone grows a very pale marigold, but no truly white marigolds have been raised.
Bill has very little contact with his brother. They see each other once in a blue moon.
Билл почти не общается с братом. Они встречаются совсем не часто.
Coin collecting is interesting, hut you find a valuable coin only once in a blue moon.
I visit my relatives once in a blue moon.
Я навещаю своих родственников очень редко.
Snow falls on the city of San Diego, California, once in a blue moon.