"Blue moons are rare, the last one occurred on March 31, 2018. There are about seven blue moons every 19 years"
This October 31, 2020 will be unique, as the sky will be illuminated by the strange blue moon.
Even though it may not be blue, the blue moon is still pretty cool. According to NASA, a blue moon is when 13 full moons occur in a year instead of the normal 12 (one each month). The 13th moon is the "blue moon", hence the term "once in a blue moon".... Makes sense.
The Halloween blue moon will be the first blue moon since March 31, 2018, and is also going to be the last blue moon to occur on Halloween until the year 2039.