Idiom
Once in a blue moon
Once in a blue moon means very rarely; something that happens once in a blue moon almost never happens.
How people use it
People use it for rare treats and habits: "We eat out once in a blue moon," or "My brother calls once in a blue moon." It usually exaggerates a little; the thing does happen, just not often. The tone is informal and friendly, and it works at the start or end of a sentence. It is not the same as "never." If you mean something will never happen, use a phrase like "when pigs fly" instead.
Examples
- We only go to the fancy steak place once in a blue moon.
- Once in a blue moon, my teenage son offers to wash the dishes.
- It snows here once in a blue moon, so the whole town shuts down when it does.
Where it comes from
In the 1500s, saying the moon was blue was a way to describe something absurd, much like claiming the moon was made of cheese. By the 1800s, "once in a blue moon" meant very rarely. Smoke or dust from volcanoes and wildfires can, on rare occasions, make the moon look bluish, which may have helped the phrase along. Today a "blue moon" also means a second full moon in one calendar month, but that sense came from a 1940s magazine article that misread an almanac, long after the idiom existed.
Check yourself
1. If your cousin visits once in a blue moon, how often does she visit?
Answer: Very rarely. Once in a blue moon means hardly ever.
2. Which sentence uses the idiom correctly?
Answer: I drink coffee once in a blue moon, maybe twice a year.. The idiom means something happens very rarely, so daily habits do not fit.
Similar idioms
Word game fan? See every word hiding in MOON.