What is the origin of the idiom-once in a blue moon?

The origin of the idiom "once in a blue moon" refers to the phenomenon of a blue moon, which is a rare event. A blue moon occurs when there are two full moons within the same calendar month, which typically happens approximately every two to three years.

The term "blue moon" originally referred to the rare occurrence of a literal blue-coloured moon, although this is extremely rare and not related to the current meaning of the phrase.

Over time, the phrase "once in a blue moon" evolved to describe something that happens very rarely or seldom, just like the occurrence of a blue moon.

So, when someone says "once in a blue moon," they mean that something happens very infrequently or rarely.