Meaning
Once in a blue moon — Something that happens very rarely or almost never.
Origin & History
A 'blue moon' historically referred to the rare occurrence of a second full moon in a calendar month. The phrase appeared in print by the 16th century meaning something absurd or impossible, and gradually shifted to mean 'very rarely'.
Example Sentences
| Sentence | Context |
|---|---|
| He calls his parents once in a blue moon. | Infrequent contact |
| Once in a blue moon, you find a truly honest politician. | Sceptical commentary |
| We only eat out once in a blue moon because of the cost. | Infrequent activity |
How to Use It
Informal to neutral. Works well in conversation and informal writing. Can be used in storytelling or light journalism.
Common Mistakes
Mistakes to Avoid
She visits once in blue moon.
She visits once in a blue moon. — The article 'a' before 'blue moon' is required.
It happens once in a blue moon every month.
It happens once in a blue moon. — 'Once in a blue moon' already implies rarity; don't add 'every month'.
Similar Idioms
Practise This Idiom
Practice English Idioms
Use these exercises to master idioms in context:
Frequently Asked Questions
What does "Once in a blue moon" mean?
"Once in a blue moon" means very rarely. Something that happens very rarely or almost never.
Where does the idiom "Once in a blue moon" come from?
A 'blue moon' historically referred to the rare occurrence of a second full moon in a calendar month. The phrase appeared in print by the 16th century meaning something absurd or impossible, and gradually shifted to mean 'very rarely'.
Can you give an example of "Once in a blue moon" in a sentence?
Here is an example: "He calls his parents once in a blue moon." — used when infrequent contact.
Is "Once in a blue moon" formal or informal?
Informal to neutral. Works well in conversation and informal writing. Can be used in storytelling or light journalism.
What CEFR level is "Once in a blue moon"?
This idiom is typically taught at B1 level. It is a intermediate expression that appears in a wide range of spoken and written English contexts.
What are common mistakes with "Once in a blue moon"?
The most common mistake is getting the articles wrong. Always use the fixed form of the idiom without changing the words. For example: She visits once in a blue moon. — The article 'a' before 'blue moon' is required.
What idioms are similar to "Once in a blue moon"?
Similar idioms include: Rarely / Hardly ever, Once in a while, Seldom. These share the same general meaning but may differ slightly in tone or context.
How do I practise idioms like "Once in a blue moon"?
LexFizz's Flip Tiles and Flash Cards exercises are great for practising English idioms. Try to write your own example sentences using the idiom in context, and listen for it in films, podcasts, and conversations.
Can "Once in a blue moon" be used in writing?
Yes, but it depends on the register. Informal to neutral. Works well in conversation and informal writing. Can be used in storytelling or light journalism. Idioms generally work best in spoken English or informal writing; avoid them in formal academic or professional documents unless you are sure of your audience.
Does "Once in a blue moon" have the same meaning in British and American English?
Yes, "Once in a blue moon" is understood in both British and American English with the same meaning: very rarely. Some idioms vary between dialects, but this one is widely recognised in all major varieties of English.