They call it a blue moon, but it has nothing to do with the colour. It's really a reference to its rarity.
What is even rarer is for there to be a blue moon on Halloween, says Jack Burnett, managing editor for The Old Farmers Almanac.
The last one was 19 years ago on Oct. 31, 2001 according to Burnett.
The blue moon is the second full moon in the same month.
"It only comes up once, sometimes twice a year," said Burnett.
"The moon has always been a mysterious orb in the sky, and it's always been felt to have sort of mysterious powers," Burnett said.
That obsession with the moon goes back to Celtic traditions and the early Christian All Hallows Day celebrated on Nov. 1. The night before, All Hallows Eve, is believed by many to originally be a Celtic harvest festival, which gave the moon great relevance in daily life.
"So because... the moon has always been so mysterious, it's been associated with Halloween.
And of course, now we see that we have the full hunter's Blue Moon on Saturday night, it makes it all the more spookier and you know, all the more Halloweenier," Burnett said.
Traditionally, hunters would be out hunting for food this time of year using the light of the moon to help them.
Ever wonder where the saying once in a blue moon comes from? According to Burnett it comes from an old English word that meant betrayed.
"So that meant that they felt it had betrayed the normal cycles of the moon by showing up, you know, an extra time. So that's one theory as to how the actual word came about."
And is it blue? No, not unless there is ash or something else in the air to make it appear blue, Burnett said.
In addition to the rare full moon on Halloween, don't forget that early Sunday morning, clocks will also fall back one hour.