Since I have never seen one, I understand why a true blue moon is considered to be rare. Although there are certain atmospheric events, like dust clouds from a volcano which can literally turn the moon blue, these are very, very rare. There are multiple meanings in common usage that are not quite so rare. We will cover the two most common.
Sometimes this refers to the fourth full moon in a calendar quarter. This happens because full moons occur every 29.5 days. The next one will be in December 2020. The moons for that quarter are Oct. 1, Oct. 31, Nov. 30, and Dec. 30; four full moons in a single quarter.
This also is the timing for an alternative definition: two full moons in one month. This happens in October 2020, so the blue moon will be Oct. 31. So, our next blue moon will be on Halloween.
Sadly, the observatory remains closed due to COVID-19.