Once in a blue moon means almost never. It describes the event that happens a few times in a lifetime, or so rarely that you cannot predict it.
The idiom gets its name from a real astronomical oddity. A blue moon is rare by definition, which makes it the perfect measure of a rare event.
This page explains the meaning, the grammar, the blue moon history and the close alternatives. The 10-question drill at the end explains every answer.
What it means
In one sentence: Once in a blue moon means very rarely, on rare occasions only.
What the idiom means in everyday English
- It means rare: "He visits once in a blue moon."
- The event is unusual but possible, unlike an impossible thing, which is "when pigs fly".
- The phrase usually answers how often: once in a blue moon means not often at all.
- It can describe people, habits, events and chances: once in a blue moon, a film like this appears.
How the idiom behaves in a sentence
- The phrase is a frequency expression, used like once a month: "I eat there once in a blue moon."
- It does not change with tense. The verb around it changes: "She calls once in a blue moon", "we met once in a blue moon".
- Common patterns are once in a blue moon and only once in a blue moon.
- The article stays a: once in a blue moon, never once in the blue moon.
When the idiom does not fit
- Do not use it for something impossible; say "when pigs fly" or "not in a million years".
- It is not for something frequent. If the event happens often, use every now and then or once a week.
- Avoid it in formal writing about schedules; write "very rarely" or "infrequently".
- Do not use it about the moon itself in a science lesson; that is the literal blue moon.
Where it comes from
A blue moon is a real but rare event: in modern use it is a second full moon within one calendar month. English has used blue for something absurd or rare since the 1500s, and the phrase once in a blue moon is recorded from the 1800s. The astronomy and the idiom grew side by side.
Use it or not
Register: Neutral to informal. Once in a blue moon is common in conversation, in writing and even in journalism. It is fine in most contexts. A formal schedule would say "very rarely", but the idiom is not wrong in a friendly email. It is never rude.
Examples
- I only eat fast food once in a blue moon.The phrase describes a rare habit.
- She visits her old school once in a blue moon.The idiom answers the question of frequency.
- Once in a blue moon, a film comes along that everyone loves.The phrase introduces a rare event.
- We meet for lunch once in a blue moon, but we talk every week.The idiom contrasts with a frequent activity.
- He checks his email once in a blue moon, so call him instead.The rarity explains the advice that follows.
- It snows in this city once in a blue moon.The idiom suits a rare weather event.
- Only once in a blue moon does she miss a deadline.Only strengthens the sense of rarity.
Similar idioms
once in a while
sometimes, but not often; more frequent than once in a blue moon.
hardly ever
almost never, the plain phrase.
few and far between
rare and not close together.
rare as hen’s teeth
extremely rare, humorous.
every now and then
occasionally; more often than once in a blue moon.
Practice drill
Answer all 10 questions. Each answer is marked at once, and the meaning is explained under the question.
I do not see him often. He calls ___.
Show answer and explanation
Answer: A. once in a blue moon Once in a blue moon means very rarely, which matches the infrequent calls.
Which sentence uses once in a blue moon correctly?
Show answer and explanation
Answer: B. She eats at that restaurant once in a blue moon. The idiom means rarely, so the first sentence fits; the others contradict the meaning or break the fixed article a.
What does once in a blue moon mean?
Show answer and explanation
Answer: C. very rarely Once in a blue moon means on rare occasions, not never and not often.
How often does something happen if it happens once in a blue moon?
Show answer and explanation
Answer: D. almost never The idiom describes an event so rare that it is almost never, like a blue moon itself.
Which event fits once in a blue moon?
Show answer and explanation
Answer: A. snow in a warm city Snow in a warm city is rare, which is what the idiom expresses; the other events are regular.
Choose the sentence with the same meaning: "He visits once in a blue moon."
Show answer and explanation
Answer: B. He hardly ever visits. Once in a blue moon means very rarely, so "hardly ever" expresses the same idea.
Which is correct?
Show answer and explanation
Answer: C. It happens once in a blue moon. The fixed phrase keeps the article a and the singular moon: once in a blue moon.
A friend asks how often you go to the theatre. It is maybe once every two years. What can you say?
Show answer and explanation
Answer: D. Once in a blue moon. Once in a blue moon is the natural answer for an event that happens only every year or two.
Which sentence does NOT use the idiom correctly?
Show answer and explanation
Answer: A. The sun rises once in a blue moon. The sun rises every day, so the idiom for rare events cannot describe it.
A colleague says, "I travel abroad once in a blue moon." What does she mean?
Show answer and explanation
Answer: B. She travels abroad very rarely. Once in a blue moon means very rarely, so her trips abroad are infrequent but still possible.
Questions learners ask
Does once in a blue moon mean never?
No. It means very rarely, but the event is still possible. For something truly impossible, use "when pigs fly" or "not in a million years".
What is a blue moon?
In modern use, a blue moon is the second full moon in one calendar month. Because full moons are about 29.5 days apart, two in one month is rare, which gives the idiom its meaning. The moon does not actually turn blue.
Is once in a blue moon formal or informal?
It is neutral to informal. It fits conversation, friendly emails and journalism. In a formal schedule, write "very rarely" or "infrequently".
More idioms
Bite the bullet
To bite the bullet is to accept an unpleasant job or a hard decision and do it, without waiting any longer.
Cost an arm and a leg
If something costs an arm and a leg, it costs a lot of money, more than it should.
Burn the midnight oil
To burn the midnight oil is to stay up late working or studying, long after the normal day has ended.
Browse all the idiom pages or take the free A1 to C2 level test to find your starting point.