What does 'once in a blue moon' mean?

What it means: Very rarely. Something that happens "once in a blue moon" almost never happens. It implies infrequency - not impossibility, but close.

Where it comes from: 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.

• "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."

The tone: 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).

• "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

Register: Casual. Works well in everyday conversation.

Tags: idiom, frequency, rarity, everyday English