October 31st full moon is called the Blue Moon because it's the second full moon of the same month---following the harvest moon of October 1st. And in a rare treat the 2020 Halloween full moon will be visible to the entire world, rather than just parts of it, for the first time since World War 11!
Interesting fact about the visibility. I read that for some parts of the world this full moon occurred on November 1st so the blue moon will be the one at the end of November.
