Once in a Blue Moon
Meaning: To do something “once in a blue moon” is to do it very rarely.
Origin: The saying has everything to do with astronomy. A “blue moon” is an astronomical phenomenon, when the second full moon happens in the same month.
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It’s a rare occurrence that happens once every 2.7 years, hence the meaning of the phrase. Usually, a blue moon will look gray or white like any other full moon. But there are rare occasions when the moon actually looks blue. According to NASA, during volcanic eruptions or forest fires, the oils in the smoke can make the moon look blue.