Very rarely; almost never; used to describe something that happens extremely infrequently.
A 'blue moon' traditionally refers to the second full moon in a calendar month, which occurs roughly every 2.7 years. The phrase evolved from earlier meanings of 'blue moon' that indicated impossibility or absurdity, since the moon rarely appears blue to the naked eye.
The astronomical accuracy makes this phrase delightfully precise - it's not just 'rarely,' but specifically every 2-3 years! Ironically, moons can actually appear blue when particles in the atmosphere scatter light, usually after volcanic eruptions or large fires, making the phrase even more wonderfully rare.
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