The second full moon to occur in a single month is called a blue moon. Every two or three years, we have one. Well, that’s the modern definition, and it’s actually based on a mistake made in 1946 in a Sky and Telescope article. There’s an older definition. Debate ensues.
The term “blue moon,” used to indicate an impossibility. These days, it more often means something is rare.
It won’t really be blue, though.
Some say this uncommon moon is auspicious (if you make plans under a blue moon, they will transpire, guaranteed). Others fear it’s foreboding (those plans may befall you in savage or unhappy ways). Some people think it’s no big deal at all. Hopefully, these folks are interesting in other ways. (Yeah, you heard me.)