Homonym: Blue Moon (Music)
A "Blue Moon" refers to an additional full moon that appears in a subdivision of a year. Traditionally, it is defined as the third full moon in a season that has four full moons, rather than the usual three. This phenomenon occurs approximately every two to three years, making it a relatively rare event in the lunar calendar.
The term has also been popularly redefined to mean the second full moon in a calendar month that contains two full moons. This modern interpretation has gained traction in popular culture, leading to phrases like "once in a blue moon," which signifies something that happens infrequently.