Once in a blue moon
The idiom “Once in a blue moon” is used to describe something that happens very rarely or almost never. It refers to the occurrence of a second full moon within a calendar month, which is quite uncommon. A blue moon is not actually blue in color but is a term used when there are two full moons in a single month, which typically happens once every 2-3 years.
The phrase "Once In a blue moon” is often used to emphasize the infrequency of an event or situation. For example, if someone says, “I only see my old college friends once in a blue moon,” they mean that they rarely get the chance to meet up with their college friends. This idiom can be applied to various situations, such as rare opportunities, unusual events, or uncommon encounters. It conveys the idea of something happening very sporadically or at long intervals.