“Once in a blue moon” means something happens very rarely or almost never.

Quick meaning

  • It’s an idiom that means “not very often,” “hardly ever,” or “on rare occasions.”
  • Example: “I eat fast food once in a blue moon” = I almost never eat fast food.

Where it comes from

  • A “blue moon” is the extra full moon that occurs rarely (like a second full moon in one calendar month, or an unusual extra full moon in a season). Because this doesn’t happen often, it became a symbol of rarity.

How people use it in sentences

  • “We go on vacation together once in a blue moon.”
  • “It only rains here once in a blue moon.”

At the end of the day, if someone says they do something “once in a blue moon,” they’re saying it’s special precisely because it doesn’t happen much. Information gathered from public forums or data available on the internet and portrayed here.