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Once in a blue moon

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LampPro Tip 1/3
Idiomatic ExpressionPlay
It's an idiomatic expression and not to be taken literally; there is no actual 'blue moon'. SlideMy aunt comes to visit us once in a blue moon, maybe every few years.
LampPro Tip 2/3
Rare OccurrencePlay
'Once in a blue moon' refers to something that happens very seldom. SlideI travel abroad once in a blue moon; it's quite a special event each time.
LampPro Tip 3/3
No Numeric FrequencyPlay
The phrase doesn't specify how often something happens, just that it's not frequent. SlideHe might play football once in a blue moon, without any regular schedule.