Once in a Blue Moon
If something happens once in a blue moon, it happens very rarely. Surprisingly, a blue moon doesn’t refer to the actual color of the moon; it refers to a full moon occurring twice a month. This happens every two to three years. It is thought that the word “blue” may have originated from the now archaic “belewe”, which meant “to betray.” It was called the “betrayer moon” because when it made its appearance, it counted as an additional spring full moon, which meant people would have to fast for an extra month during Lent. The saying as it is currently used was first recorded in 1821.