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Idiom: Once in a blue moon

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When we say that something happens once in a blue moon, what we mean is that it happens very rarely.

History of expression:

The 'moon' in this expression does, in fact, refer to the real moon when it is full.

There are generally three full moons in a season, although sometimes there may be four.

The third full moon visible on the rare occasions that there are four in a season is known as a 'blue moon'. But why blue?

The word derives from the Old English word 'belewe', which had two separate meanings. One of them was 'blue'. The other was 'betrayer'.

Many years ago, before the Gregorian calendar was reformed, the church used to calculate the date that Easter would take place that year on the cycle of the moon.

When a 'blue moon' appeared in the sky, the clergy needed to warn people that this did not represent the end of Lent and therefore bring an end to fasting, but was, in fact, a 'betrayer moon'!


"Once in a blue moon I go to the cinema, only when there's a film I really, really want to see."

"I'm very careful about what I eat so it's only once in a blue moon I eat fast food."

''Because I live abroad, I get to see my parents once in a blue moon."

Tell us about something you do once in a blue moon: