Old Farmers Almanac - It is the Almanac’s style to use Native American or colonial names for the full Moon. The first full Moon, on August 1, is the “Full Sturgeon Moon.”
The second full Moon, on August 31, is the “Full Red Moon,” which is an Algonquin name. Read our August Moon Guide for Moon phases, video, best days, folklore, and more.
This is not to be confused with the expression “blue Moon,” which many folks define as the second full Moon in a month (which occurs about every 2.5 years).
The expression "blue Moon" may have derived from the extremely rare occurrence of a Moon becoming tinged with blue when seen through atmospheric layers of forest fire smoke or volcanic dust. Since these blue-looking Moons were very unusual, the phrase “once in a blue Moon” was coined.
There was also an earlier definition of “blue Moon” related to the seasonal (tropical) calendar; the phrase’s roots have a long history involving calendars and the measuring of the year.
Nowadays, the definition that has taken off as modern folklore is simply the second full Moon in a month.
No matter what meaning you give them, blue Moons are rare, and we all know what you mean when you say, “once in a blue Moon”!
Where willows keep the lake’s green margin cool,
The speckled trout amid their shadow hides,
And dragonflies haunt every shaded pool.
– Thomas S. Collier
Sincerely, The Old Farmer’s Almanac