The Calendar year 2020 features 13 Full Moons with two Full Moons October 1 and 31. This is NOT the original definition of a Blue Moon as described below though many do refer to the second Full Moon of any month including the 2020 October 31 Full Moon as a Blue Moon.
In 2021 there are two full Moons in the sign of Aquarius on Jul 24 and Aug 22 creating 4 Full Moons in the season from the June Solstice to the September Equinox on Jun 24, Jul 24, Aug 22, and Sep 20. This is the original definition of a Blue Moon as described below.
The idea or concept that two Full Moons in a month is what defines a Blue Moon is a mistake, according to this article from the Smithsonian. The article tells us the original definition of a Blue Moon referred to 4 Full Moons in one season and a season is defined as the time between a Solstice and Equinox or an Equinox and Solstice. (See Table below) Read More