from Old Farmer's Almanac online article
A SPOOKY HALLOWEEN BLUE MOON
As stated above, October’s second full Moon appears on Saturday, October 31, making Halloween night extra special this year. This full Moon is known as the Hunter’s Moon, as this is the time of year when humans and animals alike are stocking up on food in preparation for the long winter ahead.
→ Just how rare is a full Moon on Halloween? Find out here!
Because the Hunter’s Moon is the second full Moon in the month of October, it is also considered a “Blue Moon.” This term refers either to the second full Moon to occur within a single calendar month or to the third full Moon in a calendar season. Despite the name, the Moon won’t actually appear blue-tinged; it will be its normal golden hue near the horizon, transitioning to a bright white overhead.
But wait! Here’s another fun fact that you should know about this full Moon: The Hunter’s Moon will be the smallest full Moon of the year—a so-called “minimoon“—thanks to it being farther away from Earth than any other full Moon in 2020. As you might guess, a minimoon is the opposite of a supermoon. This phenomenon is caused by the Moon’s oval-shaped orbit around Earth, which results in the Moon being farther and closer to the Earth at different points in its orbit. (The closest and farthest points in the Moon’s orbit are called perigee and apogee, respectively.)