A blue moon occurs when there is a second full moon in a single calendar month, a phenomenon that happens roughly every 2 to 3 years. Another definition, less common, is when there are four full moons in an astronomical season, and the third one is called a blue moon. The typical time between two full moons is about 29.5 days, so when a full moon happens early in a month with 31 days, there can be enough time for a second full moon in the same month, creating a monthly blue moon. Seasonal blue moons happen when a season has four full moons instead of the usual three, with the third being the blue moon. Despite the name, a blue moon does not actually appear blue in color. The next monthly blue moon will occur on May 31, 2026, and the next seasonal blue moon on May 20, 2027. Blue moons are relatively rare but predictable, occurring about every two to three years or roughly 41 times per century. February can never have a blue moon because it is too short a month to allow two full moons. In summary:

• A blue moon is usually the second full moon in a calendar month with two full moons.
• It can also mean the third full moon in a season containing four full moons.
• Blue moons happen approximately every 2-3 years.
• The next monthly blue moon is May 31, 2026; the next seasonal blue moon is May 20, 2027.
• Blue moons are not actually blue in color and are more about lunar timing than appearance.