But what exactly is it? A blue moon 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. And blue moons, which aren't actually blue in color, 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. And according to AccuWeather meteorologist Brian Lada, after this year, you won't see another blue moon on Halloween until 2039! We don't know about you, but we already can't wait.