Where did the phrase “once in a blue moon” originate and how?
Meaning: A very rare event. According to popular usage, a blue moon refers to the second full moon that occurs in any calendar month. On average, there will be 41 months that have two full moons in every century. By that calculation ‘once in a blue moon’ means once every two-and-a-half years. Origin: There are rare examples of the moon actually appearing blue, after volcanic eruptions or unusual weather conditions. That’s not what is being referred to in this phrase though. Just the opposite in fact as a blue moon was originally cited as something that was absurd and therefore impossible; only later did it come to mean unlikely.
