how often is a blue moon

A blue moon, which is typically defined as the second full moon in a single calendar month, occurs about every two to three years on average. This happens because the lunar cycle is about 29.5 days long, and most months are longer than this, allowing occasionally for two full moons in one month. Over a century, blue moons happen roughly 41 times, or about seven times every 19 years. There are also rarer types of blue moons, such as two blue moons occurring in one year, which happens only a few times per century. The phrase "once in a blue moon" thus refers to this relatively infrequent but predictable event.