What is a blue moon?

Despite the name, a blue moon has nothing to do with colour. The most common modern definition is the second full moon to occur within a single calendar month, which happens because the roughly 29.5-day lunar cycle occasionally fits twice into a longer month. An older definition refers to the third full moon in a season that contains four full moons instead of the usual three. Blue moons are uncommon, occurring on average about every two to three years, which is the origin of the saying once in a blue moon. On rare occasions the Moon really can look bluish, but that is caused by dust or smoke particles in the atmosphere from events like volcanic eruptions or large wildfires.