The common used phrase in English “Once in Blue Moon” refers to something that occurs very rarely.
The term “BLUE MOON” has nothing to do with the colour Blue of the moon.
History of “BLUE MOON”
Most years on average have 12 full moons, with 1 appearing each month.That’s because the lunar month—the time it takes the moon to cycle through its phases—corresponds closely to the calendar month.
But the calendar year is actually based on the solar cycle, or the time it takes Earth to make one trip around the sun. This means a year is not evenly divisible by lunar months, so every three years or so there are 13 full moons.
The farmer’s almanac further divided the year into four seasons, with each season lasting three months. When a given season saw four full moons, the almanac dubbed the third moon as a blue moon.
“True” Blue Moon
Before the term “blue moon” more often referred to the rare instances when the moon actually seemed to turn blue, as can happen under certain atmospheric conditions.
“After a forest fire or volcanic eruption, there may be enough particulate matter in the air so that the moon can take on a bluish tinge,”
For instance, a “true” blue moon occurred in 1950 after a large forest fire in Canada blew smoke across most of the Northern Hemisphere.
Another appeared in 1980 after the last major eruption of Mount St. Helens, which sent tons of ash into the upper atmosphere.
Information Courtesy: National Geographic News