why do they call it a blue moon

The term "blue moon" has two main meanings, both relating to the rarity of certain full moons. Traditionally, a blue moon is the third full moon in a season that has four full moons instead of the usual three. This definition comes from the practice of naming full moons by season, where occasionally one season has an extra full moon, and the third one is called the blue moon. More commonly today, "blue moon" refers to the second full moon in a single calendar month, an interpretation that started from a misinterpretation published in an astronomy magazine in the 1940s. This usage has become widespread and represents the idea of a rare event, roughly happening once every two to three years. The term originates from old expressions meaning rarity or impossibility and has sometimes been used when the moon actually appeared blue due to atmospheric effects (for example, after major volcanic eruptions causing dust in the air). However, the moon is not usually blue in color during a blue moon event. In summary, "blue moon" literally originated from rare full moon events either by calendar or seasonal count, or occasionally from the actual visual phenomenon under special atmospheric conditions, but today the phrase mostly means the second full moon in a calendar month, emphasizing rarity.