What does "once in a blue moon" mean?

Very rarely. Something that happens "once in a blue moon" almost never happens. It implies infrequency, not impossibility, but close. If a friend says they go to the gym "once in a blue moon," they are admitting they hardly ever go, though they haven't given up completely. The phrase always describes an event that does happen, just not often, so you would never use it for something that never occurs at all.

A "blue moon" is the second full moon in a single calendar month, which happens roughly every two to three years. It's rare, predictable, but infrequent, hence the phrase. Interestingly, the moon does not actually turn blue. The name refers to timing, not colour, though a moon can occasionally look bluish after volcanic eruptions or wildfires put fine particles in the air. That real-world rarity is exactly what makes the idiom stick in the mind, so if you struggle to remember it, picture a genuinely rare, faintly blue moon in the sky.

• "He calls once in a blue moon - you're lucky to hear from him twice a year."
• "We eat out, but only once in a blue moon."
• "Events like this happen once in a blue moon."
• "I treat myself to a fancy coffee once in a blue moon, maybe on a birthday."
• "Our whole family gets together once in a blue moon now that we live so far apart."

Often used with affection or mild frustration. "She visits once in a blue moon" could be said warmly (it's a special occasion) or with slight criticism (she should visit more). Context and tone of voice decide which meaning lands. In a casual chat with friends, it usually feels playful. In a workplace it can carry a gentle complaint, for example "Our manager praises us once in a blue moon," which quietly suggests the praise should come more often.

Learners sometimes say "in a blue moon" or "once in the blue moon," but the fixed form is "once in a blue moon." Keep all four words together and do not swap "a" for "the." Another slip is using it for something that truly never happens. If it genuinely never occurs, say "never," not "once in a blue moon."

• "Rarely" - neutral and direct
• "Hardly ever" - very common in spoken English
• "Almost never" - plain version
• "Once in a while" - different meaning, this means occasionally, more frequent than once in a blue moon

"Few and far between." This describes things that are rare and widely spaced, for example "Good films like that are few and far between." It overlaps with "once in a blue moon," but it describes how rare things are in general rather than how often a single event happens. Another close cousin is "every now and then," which, like "once in a while," means occasionally and is more frequent than "once in a blue moon."

The idiom is understood and used on both sides of the Atlantic with the same meaning, so you can use it comfortably in the UK, the US, Australia, and beyond. There is no real difference in tone between British and American speakers here, which makes it a safe, universal choice.

Casual. Works well in everyday conversation. It is fine in friendly emails and relaxed chats at work, but for a formal report or academic writing, choose a plainer word such as "rarely" or "infrequently" instead.