There are two definitions of a Blue Moon; both are a type of Full Moon and have nothing to do with color.
If the Moon looks blue, it’s because of dust in the atmosphere.
How Rare Is a Blue Moon?
The term once in a Blue Moon suggests something happens very rarely. So, how often do Blue Moons occur? The answer depends on how you define a Blue
There are two ways of calculating the date of a Blue Moon.
- Seasonal Blue Moon = The third Full Moon in an astronomical season with four Full Moons (versus the usual three).
- Monthly Blue Moon = The second Full Moon in a month with two Full Moons.
In the 1100 years between 1550 and 2650, there are 408 seasonal Blue Moons and 456 monthly Blue Moons. This means that either Blue Moon occurs roughly every two or three years, although the monthly ones are a little more frequent than the seasonal ones.
Blue Flower Moon in May
The seasonal Blue Moon is the original astronomical definition of a Blue Moon. Usually, there are three Full Moons between each astronomical season, which is the time between each solstice and equinox. But some years, there are four Full Moons in a season. When this happens, the third Full Moon is called a Blue Moon.
In 2019, May’s Full Moon is a seasonal Blue Moon. The astronomical season started with the equinox on March 20, and the first Full Moon alignment was less than four hours later; on March 21. The second Full Moon was on April 19, and the third, the Blue Moon, is on May 18. The fourth and last Full Moon before the June solstice is on June 17.
source – timeanddate.com