How did the expression “Once in a blue moon” came to be?
The expression ” once in a blue moon’ means rarely, almost never. If you say something happens once in a blue moon, you mean it happens very rarely because a blue moon is the second of two full moons in the same month, which happens rarely. 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. Since 1819, The Maine Farmers’ Almanac has listed the dates of forthcoming blue moons. The compilers of the almanac didn’t use the ‘second full moon in a month’ definition that is now generally accepted but instead defined a blue moon as the third full moon in a season which has four. The ‘blue moon’ expression itself is old and dates back to mediaeval England. For example, a work by William Barlow, the Bishop of Chiches
