1
general::
Rarely, once in a very long time, as in We only see our daughter once in a blue moon. This term is something of a misnomer, because an actual blue moon? that is, the appearance of a second full moon in the same calendar month? occurs every 32 months or so. Further, the moon can appear blue in color at any time, depending on weather conditions. [Early 1800s]
American Heritage Idioms
2
general::
Phrase(s): once in a blue moon
Cliché very seldom. • Jill: Does your husband ever bring you flowers? Ellen: Once in a blue moon. • Once in a blue moon, I buy a fashion magazine, just to see what people are wearing.
McGrawhill's American Idioms And Phrasal Verbs