A “Blue Moon” refers to a rare second full moon in a calendar month.
The term “Blue Moon” most commonly refers to a second full moon in a given calendar month. This occurs somewhat rarely. The next one will be in August 2023. A Blue Moon does not actually change the Moon’s appearance. The actual hue of the moon is due to particles in the air, such as ash and dust. When blue light is scattered away, this produces a red looking Moon. When the particles in the air have a specific size, red light can be scattered away, leaving mostly blue light left and the Moon appearing blue in the sky. This only happens very rarely, “once in a Blue Moon”, as the saying goes.
Above: Just a normal “Blue Moon” seen on July 31, 2015. It is not actually blue.
Below: Image of a pale, visibly blue Moon on the night of 25 December 2012.