A Blue Moon occurs about once every 2.4 to 3 years on average. It is defined most commonly as the second full moon in a single calendar month, which happens because the average time between full moons is about 29.5 days, slightly shorter than most calendar months. This leads to about 41 Blue Moons per century, or roughly 7 Blue Moons every 19 years. Another definition is the third full moon in a season that has four full moons, which happens at a similar frequency. Blue Moons do not actually appear blue; the term refers to their rarity. Occasionally, rare atmospheric conditions like volcanic ash or smoke can give the moon a blue tint, but this is unrelated to the calendar definition. February cannot have a Blue Moon since it is too short. Sometimes, two Blue Moons can occur in the same year, which is much rarer, about 3-5 times per century. The next monthly Blue Moon (second full moon in a month) will be on May 31, 2026, and the next seasonal Blue Moon (third full moon in a season with four full moons) will be on May 20, 2027.