A “blue moon” refers to the second full moon in a single calendar month, and it happens fairly regularly on average, about once every two and a half to three years. This is because the cycle of the moon’s phases is a little bit shorter than most months in our calendar.
It’s important to note that the moon itself doesn’t actually turn blue during a blue moon. The phrase “once in a blue moon” meaning something rare likely originated in the 16th century and has a separate meaning from the astronomical definition.
