Once in a blue moon

Once in a blue moon
Meaning: very rarely

True love happens once in a blue moon...

Very occasionally, the moon actually does appear to be blue. This sometimes occurs after a volcanic eruption, like that of Krakatao in 1883. Dust particles in the atmosphere are normally of a size to diffract blue light, making the moon appear reddish at sunset. Larger volcanic dust particles diffract red light, making the moon appear bluish.

Tempting as it is to suppose that something that happens very rarely, and which is mentioned by name in a phrase that means 'very rarely', is the source of the phrase, it probably isn't.

Actual examples of the moon appearing blue would in fact be the exception that proves the rule, as the 'blue moon' was originally something that was considered not rare but impossible. The two notions, 'a blue moon' and 'the moon is made of green cheese', were synonyms for absurdity, like 'pigs might fly'.

The 'blue moon' expression with the 'impossibility' meaning is old and dates back to mediaeval England; for example, a work by William Barlow, the Bishop of Chichester, the Treatyse of the Buryall of the Masse, 1528, included a sarcastic reference to a blue moon: