when does a blue moon happen

A blue moon happens when there is a second full moon within a single calendar month or when there is a third full moon in a season that has four full moons instead of the usual three. The two types of blue moons are called monthly blue moons and seasonal blue moons. Monthly blue moons occur roughly every 2.5 to 3 years when two full moons happen in one month. Seasonal blue moons happen when a season has four full moons, and the third is called a blue moon. The next monthly blue moon will be on May 31, 2026, and the next seasonal blue moon will be on May 20, 2027. The term "blue moon" does not refer to the actual color of the moon, which is generally gray or white; the moon appearing blue is a rare atmospheric phenomenon unrelated to this timing event.