Rare Blue Moon 2024 Calendar. But it won't be just any old full moon, it will be a supermoon blue. When was the last blue moon?

The saying once in a blue moon refers to something that’s incredibly rare, and indeed, these celestial events occur roughly every two or three years. Stargazers will be treated to a “blue supermoon” on monday night, with the rare event expected to be visible thanks to clear skies.