Once in a blue moon is an idiom that goes back to medieval times, though the meaning has changed from the original. We will look at the definition of once in a blue moon, where the term comes from, how the meaning has changed and some examples of its use in sentences.
Once in a blue moon means rarely, seldom, not happening very often. The idiom is taken from a common term for an astronomical event. A blue moon is the second full moon that occurs within a calendar month. Since lunar months and calendar months are of differing durations, a blue moon occurs approximately every two years and eight months. Though the moon may appear blue at some times because of atmospheric variables such as volcanic dust, blue moons are not actually blue. Interestingly, the original meaning of blue moon described something that was impossible, something that never happened. In the early part of the nineteenth century, blue moon came to mean something that happened rarely. The plural form of blue moon is blue moons. Once in a blue moon is an idiom, which is a phrase that is used figuratively and is generally not to be taken literally.
“We live in such an amazing place in terms of nature and waterways and we didn’t want to miss out on something that happens once in a blue moon,” Dearman said. (The East Bay Times)
But when, once in a blue moon, we used it for organising a function of the Library, the other businessmen took objection. (The Free Press Journal)
Even if raids are conducted for the heck of it once in a blue moon, the names of the offenders are never made public. (Greater Kashmir)