What is a Blue Moon and how often does it happen?
Quick Answer: A Blue Moon is the second full moon in a single calendar month. It happens about once every 2.7 years, making it a rare but not impossible event.
The term "Blue Moon" originally referred to the third full moon in a season with four full moons, but now it mostly means the second full moon in one month. Since most months have only one full moon, having two is unusual.
This happens because the lunar cycle is about 29.5 days, slightly shorter than most months. When timing aligns just right, you get two full moons in one month, creating the Blue Moon.