once in a blue moon

once in a blue moon

Very rarely. Peter only comes out for a drink once in blue moon now that he has kids. A: "Do you ever eat pork?" B: "Only once in a blue moon. I prefer beef."
See also: blue, moon, once
Farlex Dictionary of Idioms.

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.
See also: blue, moon, once
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

once in a blue moon

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]
See also: blue, moon, once
The American Heritage® Dictionary of Idioms by Christine Ammer.

once in a blue moon

If something happens once in a blue moon is happens only very rarely. I only get over to Cambridge once in a blue moon and I'm never in London. Only once in a blue moon do properties of this quality become available. Note: In some places, if there are two full moons in a calendar month, the second is called a `blue moon'. This is rare.
See also: blue, moon, once
Collins COBUILD Idioms Dictionary, 3rd ed.

once in a blue moon

very rarely; practically never. informal
The colour blue was an arbitrary choice in this phrase. To say that the moon is blue is recorded in the 16th century as a way of indicating that something could not be true.
See also: blue, moon, once
Farlex Partner Idioms Dictionary

ˌonce in a blue ˈmoon

(informal) very rarely: Sue’s daughter only visits her once in a blue moon.
See also: blue, moon, once
Farlex Partner Idioms Dictionary

once in a blue moon

mod. rarely. Once in a blue moon I have a little wine with dinner.
See also: blue, moon, once
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions

once in a blue moon

Very rarely; once in a long while. The earliest reference to a blue moon is skeptical, as well it might be: “Yf they saye the mone is blewe, we must believe that it is true” (Roy and Barlow, Rede Me and Be Not Wrothe, 1528). An early reference to the moon as a time period occurred in Thomas Dekker’s A Knight’s Conjuring (1607): “She would have trickes (once in a moone) to put the divell out of his wits.” The pairing of the two came only in the nineteenth century: “That indefinite period known as a ‘blue moon’” (Edmund Yates, Wrecked in Port, 1869). It occurred still earlier in Pierce Egan’s Early Life in London (1821): “How’s Harry and Ben? Haven’t seen you this blue moon.” Modern astronomers now say “blue moon” refers to the occurrence of a fourth full moon in a season, in addition to the usual three.
See also: blue, moon, once
The Dictionary of Clichés by Christine Ammer
See also:
  • blue moon
  • out of a clear (blue) sky
  • out of the blue
  • blue screen of death
  • BSOD
  • gown
  • blue gown
  • curse a blue streak
  • have a blue fit
  • cuss a blue streak
References in periodicals archive
After all, it happens much more often than once in a blue moon.
Stargazing; Supermoon returns for a blockbuster midnight showing
"The second full moon is called a blue moon, it happens once every three years, it's a rare occasion and that's where the expression "once in a blue moon" comes from to describe something that doesn't happen very often.
Eclipse of 'blue moon' tonight..
There are several definitions of a Blue Moon but folklore has it that it is the occurrence of a second full moon in a single calendar month, which happens only once every two or three years, hence the term "once in a Blue Moon" being used to describe rare events.
Once in a Blue Moon
THE phrase "once in a blue moon" is frequently used to refer to an event that occurs very rarely.
YOU ASK & WE ANSWER
At the end of the month, put OI' Blue Eyes on the turntable, whip up a batch of these drinks (each garnished with a moonlike slice of orange), and lift a glass to the kind of warm summer evening that only happens once in a blue moon.
Cocktail: blue moon
The phrase 'once in a blue moon' meaning something rare or unusual came as a result of the vast eruption of the volcano on the Pacific island of Krakatoa.
Archive: On This Day
Some three years later, in March 1946, an article entitled "Once in a Blue Moon" appeared in Sky & Telescope (page 3).
Whats a Blue Moon?