Something will happen Saturday, Oct. 31, that occurs only once in a blue moon.

That’s it, a “blue moon.”

The blue moon, while normally not blue in color, will be the second full moon in October.

Once in a blue moon has been an expression for years to describe an event that doesn’t reoccur very often.

The website EarthSky explains:

The idea of a Blue Moon as the second full moon in a month stemmed from the March 1946 issue of Sky and Telescope magazine, which contained an article called Once in a Blue Moon by James Hugh Pruett. Pruett was referring to the 1937 Maine Farmer’s Almanac, but he inadvertently simplified the definition. He wrote:

Seven times in 19 years there were – and still are – 13 full moons in a year. This gives 11 months with one full moon each and one with two.

This second was called Blue Moon.

There’s another definition for Blue Moon. It can be the third of four full moons in a single season, with a season being between a solstice and equinox. The next seasonal Blue Moon will be August 22, 2021.

The moon can appear blue when there is a lot of junk (dust particles or smoke) in the air, but it’s not the classic blue moon.

The time between one full moon and the next is close to the length of a calendar month – or “moonth” (sic). So the only time one month can have two full moons is when the first full moon happens in the first few days of the month. This happens every two to three years, so these sorts of Blue Moons come about that often.

Very rarely, a seasonal Blue Moon (third of four full moons in one season) and a monthly Blue Moon (second of two full moons in one calendar month) can occur in the same calendar year.

For this to happen, you need 13 full moons between successive December solstices for a seasonal Blue Moon – and, generally, 13 full moons in one calendar year for a monthly Blue Moon.