Blue moons occur roughly every two to three years. This frequency applies to both types of blue moons:

• Monthly Blue Moon: This is the second full moon in a single calendar month. Since the lunar cycle is about 29.5 days, months with 31 days can occasionally have two full moons, making the second one a blue moon. This happens about seven times every 19 years, or roughly once every 2-3 years

• Seasonal Blue Moon: This occurs when there are four full moons in a single astronomical season (instead of the usual three), and the blue moon is the third full moon of that season. Seasonal blue moons also happen about seven times every 19 years

On average, a blue moon occurs once every 33 months (about 2.75 years)

. Sometimes, two blue moons can occur in the same calendar year, but this is rare-about three to five times per century

. The moon does not actually turn blue during a blue moon; the term relates to timing, not color. True blue-colored moons are caused by rare atmospheric conditions like volcanic ash or smoke, not by the lunar cycle itself