Classic depictions of Halloween often show a bright full moon in a sky full of witches and bats – and this year, part of that iconography is correct. The Moon will be bright and full on the eve of All Hallows.
The event will also be a Blue Moon, the name given to the second moon in a calendar month. The Moon won’t actually turn blue unless you are close to a volcanic eruption or a wildfire, which can release enough ash to give the Moon an unusual bluish tint. If you are in such a scenario, please get to safety before looking at the sky.
Blue moons are rare but not unheard of, hence the saying "once in a blue moon". They occur because there are 12.37 lunar revolutions around the Earth every year. This means that every 2.5 to 3 years we get an extra lunar cycle, including an extra full moon. A lunar cycle, also known as a lunation, lasts 29.53 days, so given the fact that October has 31 days, a full moon that falls on Halloween is always a blue moon. However, an alternative definition is the third moon of a season with four moons, which would make it impossible to have a blue moon on Halloween.
The Moon will be full at 10:49 am EDT (2:51 pm UTC). You might want to take a peek, as the full moon on Halloween follows the Lunar Metonic cycle, which is 235 full moons, so takes place only every 19 years. The next Halloween full moon will occur in 2039, then 2058, and 2077.
However, due to the slight differences in precise timing, leap years, etc, the pattern can break. We had no Halloween full moon in 2000, 1982, or 1963 (though we did in 1944). Those full moons were always a day ahead or behind. Still, the Moon looked big and bright, even if it was not perfectly full.
The last full moon we had on Halloween also didn't fit the 19-year pattern, which occurred on October 31, 1974. That means this Full Hunter's Moon has been 46 years coming. For a very rare event, maybe we should start saying once in a Halloween Blue Moon!