The idiom once in a blue moon means something happens very rarely.
The phrase refers to a blue moon, a name for an extra full moon that appears in a calendar cycle. Because it does not occur often, the image became a way to describe rare events.
The phrase refers to a blue moon, a name for an extra full moon that appears in a calendar cycle. Because it does not occur often, the image became a way to describe rare events.
We only see each other once in a blue moon.