A blue moon, defined as the second full moon in a single calendar month, occurs on average about once every 2.4 to 2.7 years. This means it typically happens roughly every two to three years. In terms of frequency, that amounts to about 41 blue moons per century or about seven times every 19 years. The occurrence depends on the lunar cycle of about 29.5 days and the length of calendar months, especially favoring months with 31 days. The blue moon does not actually appear blue; it is simply a timing phenomenon of full moons within calendar months. Occasionally, there can be two blue moons in a single year, which is rarer and happens a few times per century.
