Idiom: Once in a blue mood
OED definition: Very rarely
Example: He prefers to stay at home and only goes out once in a blue moon
The meaning of ‘once in a blue moon’ has changed slightly over the years as it used to mean that something was ridiculous or absurd. In fact, the original meaning was more akin to today’s use of ‘when pigs fly’.
The first recorded use of the phrase with its original meaning was in a pamphlet published by William Roy and Jeremy Barlow in 1528. In a conversation between two of the characters, reference is made to the fact that priests (and the Church in general) would make different statements and expect the average person at the time to believe everything that they were told was true, no matter how ridiculous it seemed.
Not until 1821 did we see the idiom being used with its current meaning of ‘rarely’. Since Pierce Egan included it in a dialogue between two of the characters in his novel Real Life in London, it has become a well know saying.
But what is a blue moon anyway? Usually, each calender month has one full moon, however, every two or three years there is a month which has two. The second of these full moons is referred to as a ‘blue moon’. As a blue moon does not happen very often, this is the reason why this idiom has taken on its current meaning of ‘rarely’.
If you are interested in seeing a blue moon (although don’t expect it to actually look blue) have a look at the skies on 21st May 2016 as this is when the next one is set to occur.