how often do blue moons happen

Blue Moons happen roughly every two to three years on average. This happens because a Blue Moon is defined as the second full moon occurring in a single calendar month, and with the lunar cycle being about 29.5 days, occasionally two full moons fit within one month. Another type, a seasonal Blue Moon, happens when there are four full moons in a season instead of the usual three. Both types of Blue Moons occur about every two to three years, with a typical frequency of around 7 times every 19 years or 41 times per century. February cannot have a Blue Moon due to its short length. Additionally, Blue Moons are named for this timing phenomenon and not because the Moon actually appears blue, which is a rare atmospheric effect unrelated to the calendar definition.