Have you ever heard someone use the phrase"once in a blue moon?" People use this expression to describe something that they do not do very often. For example,someone might say that he tries to avoid eating sweets because they are unhealthy,but will eat chocolate"once in blue moon." Or someone who does not usually like to go the beach might say"I visit the shore once in a blue moon." While many people use this phrase,not everyone knows the meaning behind it.