- Very rarely; almost never.
Explanation
Origin
Two possible origins, often conflated:
- 1. The astronomical event: A blue moon is the second full moon occurring within a single calendar month (or sometimes the third of four full moons in a season). This is relatively rare, happening roughly every 2-3 years.
- 2. An older, more figurative meaning: Dating back centuries, blue moon might have simply meant something absurd or impossible, similar to when pigs fly. The earliest recorded usage (16th century) suggests it meant never.
The modern understanding leans towards very rarely, influenced by the astronomical definition's relative frequency.
Synonyms & Related Expressions
Alternatives
Slang/Informal:
- Like, never. (Exaggeration)
- Hardly.
- Not often.
Exaggerated/Figurative (meaning almost never or never):
- When pigs fly.
- Not in a million years.
- Donkeys' years ago (UK/Aus referring to the last time something happened)
Situational Appropriateness
- Informal to semi-formal. Widely understood and acceptable in most contexts except the most formal writing.
Misunderstanding Warnings
- Some learners might be confused by the literal color blue or the astronomical specifics, but the meaning of rarely is usually clear from context.
Examples
- I only eat fast food once in a blue moon.
- We live so far apart, we only see each other once in a blue moon.
- An opportunity like this comes along once in a blue moon.
Dialogue
Alex: Do you ever go back to visit your hometown?
Sam: Once in a blue moon, maybe. It's a long drive, and everyone I knew has mostly moved away.
Alex: Ah, I see. Yeah, that makes sense.
Social Media Examples
- Tweet: Decided to treat myself to a fancy dinner. Only happens once in a blue moon! #treatyourself #rareoccasion
- Instagram caption: Saw a shooting star tonight! ✨ Happens once in a blue moon. Made a wish! #stars #nightsky
Response Patterns
- Acknowledgment: Ah, so not very often then.
- Expressing surprise at the rarity: Really? Only that often?
- Relating: Yeah, same here.
Common Follow-up Questions/Actions
After hearing something happens once in a blue moon:
- Asking why it's so rare: Why so seldom?
- Asking about the last time: When was the last time you did?
- Expressing a wish for more frequency: That's a shame, it would be nice if it happened more often.
Conversation Starter
- Can be used to start a conversation about frequency. How often do you go to the theater? Oh, once in a blue moon.
Intonation
- Emphasis often on ONCE and BLUE MOON.
- Can be delivered factually or with a tone emphasizing the rarity. ONCE in a BLUE MOON.
Generation Differences
- Common and understood across all generations.
Regional Variations
- Used universally in English-speaking countries.