Once in a blue moon: Get ready for a Halloween full moon - USA TODAY

For the first time in decades, there will be a full moon visible across the entire U.S.

For the first time in decades, there will be a full moon visible across the entire U.S.

The last time that a full moon was seen in all U.S.

The Halloween full moon is also known as a "blue" moon because it's the second full moon of the month.

Fun fact: Every full moon on Halloween is a blue moon, because moon cycles are 29.5 days long and Halloween is always on Oct

(There is an alternate definition of a blue moon: When there are four full moons in a single season, the third is considered a blue moon, the Farmers Almanac said.) 

In addition to being called a blue moon, Halloween's full moon is also called the Hunter's moon. "Many moons ago, Native Americans named this bright moon for obvious reasons," the Farmer's Almanac said. "The leaves are falling from trees, the deer are fattened, and it’s time to begin storing up meat for the long winter ahead."

The first full moon of the month, on Oct