The night sky on Halloween will be illuminated by a blue moon, the second full moon in a month. The relatively rare occurrence happens once every two and a half years on average, according to NASA's National Space Science Data Center.
Every month has a full moon, but because the lunar cycle and the calendar year aren't perfectly synched, about every three years we wind up with two in the same calendar month.
October's first full moon, also known as the harvest moon, will appear on the first day of the month. The second full moon, or blue moon, will be visible on October 31. It's the first instance of a blue moon in the Americas since March 2018>>> Read more here>>https://6abc.com/science/a-rare-blue-moon-will-light-up-the-sky-on-halloween/6672894/
Source: 6 abc.com