A Blue Moon, defined as the second full moon occurring within a single calendar month, happens about every 2.4 to 3 years on average. This averages roughly 41 Blue Moons per century. Another way to say it is that a Blue Moon occurs once every 33 months or about seven times every 19 years. There are also two types of Blue Moons: the monthly Blue Moon (the second full moon in a calendar month) and the seasonal Blue Moon (the third full moon in an astronomical season with four full moons). Both types occur roughly every two to three years, with variations due to calendar and time zone differences. Despite the name, a Blue Moon does not actually appear blue in color except very rarely due to atmospheric conditions like volcanic ash or smoke. In summary:

• A Blue Moon occurs roughly every 2 to 3 years.
• There are typically 41 Blue Moons per century.
• The term usually means the second full moon in a calendar month.
• A Blue Moon does not literally look blue under normal conditions.

This explanation holds for the most common and modern understanding of a Blue Moon.