Get ready for an extraspooky Halloween [1] this year, because the astronomical phenomenon [2] known as a blue moon is happening on none other than Oct. 31.

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 [3], so there's usually only one full moon per month. And blue moons, which aren't actually blue in colour, occur only once every two and a half to three years [4], according to NASA.

Though a full moon might not seem so special, a full moon that falls on Halloween [5] only happens three or four times a century [6]. And according to AccuWeather meteorologist Brian Lada, after this year, you won't see another blue moon on Halloween until 2039 [7]! We don't know about you, but we already can't wait.