A blue moon happens when there's a fourth full moon in a single season. Usually, there are only three full moons in each season, so the blue moons rarely happen. Since it's such a rare event, we use the phrase "once in a blue moon" to describe something that doesn't happen very often. For example, here's some dialogue - Bob: "How often do you read the news?" Alice: "Every morning. And you?" Bob: "I only read it once in a blue moon." Hope this helped!