Once in a Blue Moon

Expression: "Once in a blue moon."

Meaning: The origin of this expression is not actually English, but a non-English word that sounds like the color blue. This word was used to name the second full moon on those rare occasions that two full moons occurred during one calendar month. Because the phrase sounded like "blue moon," it became "blue moon." And because blue moons are rare, this idiom evolved to mean something that happens rarely.

Example: "Since having children, they now only go out once in a blue moon."