blue moon

a long or seemingly long period of time once in a blue moon we'll get dressed up and eat in a fancy restaurant Synonyms aeon (or eon), age, coon's age, cycle, donkey's years [chiefly British], eternity, forever, long, months, moonRelated Words infinity; lifetimeNear Antonyms flash, instant, jiffy, minute, moment, second, shake, split second, trice, twinkle, twinkling, wink; microsecond, nanosecond