But what exactly is a blue moon.  A blue moon isn’t actually blue, it refers to the second full moon in one month, and it’s pretty rare (hence the phrase “once in a blue moon”). A full moon occurs every 29 days, so there’s usually only one full moon per month. Blue moons, occur only once every two and a half to three years, according to NASA.

Though a full moon might not seem so special, a full moon that falls on Halloween only happens three or four times a century.  After this year, you won’t see another blue moon on Halloween until 2039! We don’t know about you, but we can’t wait.