This weekend, a full moon will rise in the night sky, a so-called "Blue Moon." 

Typically, a Blue Moon is defined as the second full moon that occurs during a calendar month, but the full moon this Saturday (May 21) will be the only full moon of May 2016. So, how can it be called a Blue Moon?

The explanation points back to a somewhat obscure rule. In fact, the current rule of two full moons in one month has superseded the rule that would allow this month's full moon to be called "blue."

If you're confused, don't worry. Here's the rest of the story. 

The almanac rule

In a question-and-answer column from the July 1943 issue of Sky & Telescope magazine, writer Lawrence J. Lafleur referenced the term "Blue Moon." Lafleur cited the unusual term from a copy of the 1937 edition of the now-defunct "Maine Farmers' Almanac" (not to be confused with the "Farmers' Almanac," which is still published in Lewiston, Maine). 

On the page for August 1937, the "Maine Farmer's Almanac" gives the calendrical meaning for the term "Blue Moon."

That explanation said that the moon "usually comes full 12 times in a year, three times for each season." Occasionally, however, there will come a year when there are 13 full moons during a year, not the usual 12. 

The almanac explanation continued: "This was considered a very unfortunate circumstance, especially by the monks who had charge of the calendar of thirteen months for that year, and it upset the regular arrangement of church festivals. For this reason, thirteen came to be considered an unlucky number."

And that extra full moon also meant that one of the four seasons would contain four full moons instead of the usual three. 

"There are seven Blue Moons in a lunar cycle of 19 years," the almanac said, ending on the comment that, "In olden times, the almanac makers had much difficulty calculating the occurrence of the Blue Moon, and this uncertainty gave rise to the expression 'once in a Blue Moon.'" 

An unfortunate oversight

While LaFleur correctly quoted the almanac's account, he made one important omission: He never specified any date for the Blue Moon.And as it turned out, in 1937, the Blue Moon occurred on Aug. 21. This was the third full moon in the summer of 1937, a summer season that would see a total of four full moons. 

Names were assigned to each full moon in a season. For example, the first moon of summer was called the early summer moon, the second was the midsummer moon, and the last was called the late summer moon. But when a particular season has four moons, the third was apparently dubbed a "Blue Moon," so that the fourth and final one can continue to be called the late moon.

So where did the two-full-moons-in-a-month rule that is so popular today come from?

Pruett's mistake

Once again, the answer hails from the pages of Sky & Telescope magazine. This time, on Page 3 of the March 1946 issue, James Hugh Pruett wrote an article titled "Once in a Blue Moon." Here, he used the term "Blue Moon" and referenced LaFleur's article from July 1943. But because Pruett had no specific dates to fall back on, his interpretation of the ruling given by the "Maine Farmers' Almanac" was highly subjective. 

Pruett ultimately came to the following conclusion: "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 in a month, so I interpret it, was called Blue Moon."  

It's unfortunate that Pruett did not have a copy of that 1937 almanac at hand, or else he almost certainly would have noticed that his two-full-moons-in-a-single-month assumption was wrong. That's because the Blue Moon date of Aug. 21 was notthe second full moon that month.

Going viral

Pruett's 1946 explanation was, of course, wrong, and it might have been completely forgotten were it not for science journalist Deborah Byrd. She cited Pruett's interpretation on the Jan. 31, 1980, episode of her popular National Public Radio program "StarDate." It could be said that in the aftermath of her radio show, the incorrect Blue Moon rule "went viral."

Over the next decade, this new Blue Moon definition appeared in such diverse places such as the kids' edition of "The World Almanac" and the board game "Trivial Pursuit."

I must confess that even I helped perpetuate the new Blue Moon phenomenon. Nearly 34 years ago, in the Dec. 1, 1982, edition of The New York Times, I made reference to the erroneous Blue Moon explanation in the "New York Day by Day" column. 

And by 1988, the new definition had started receiving international press coverage. 

Today, Pruett's misinterpreted two-full-moons-in-a-month rule is recognized worldwide. Indeed, Sky & Telescope turned a literary lemon into lemonade, proclaiming later that the magazine had, however unintentionally, changed pop culture and the English language in unexpected ways.

Meanwhile, the original "Maine Farmers' Almanac" rule has been all but forgotten.

Playing by the (old) rules

Now, let's come back to this month's full moon. 

Under the "old" almanac rule, this Saturday's moon would be a Blue Moon.

In spring 2016, there are four full moons, occurring on March 23, April 22, May 21 and June 20.

June 20, 2016, is the first day of summer if you live north of the equator, but south of the equator, that date is the first day of winter. In 2016, the summer solstice comes at 2234 GMT or 6:34 p.m. ET on June 20. But the moon turns full at 1103 GMT or 7:03 a.m. ET. That's 11 hours and 31 minutes before the solstice occurs. So the June 20 full moon occurs during the waning hours of spring and qualifies as the fourth full moon of the season. 

This means that under the original "Maine Almanac" rule — the one promoted by Lafleur and later misinterpreted by Pruett — the third full moon of the 2016 spring season on May 21 would be a Blue Moon. 

Final thoughts

So which Blue Moon definition tickles your fancy? Is it the second full moon in a calendar month or (as is the case on Saturday) the third full moon in a season with four? Maybe it's both. The final decision is solely up to you.

Saturday's full moon will look no different than any other full moon — it won't likely be blue. But the moon can take on such a color in certain conditions. After forest fires or volcanic eruptions, Earth's satellite can appear to take on a bluish or even lavender hue. Soot and ash particles, deposited high in the Earth's atmosphere, can sometimes make the moon appear bluish.

For instance, in the aftermath of the massive eruption of Mount Pinatubo in the Philippines in June 1991, there were reports of a blue-colored moon (and even a blue sun) worldwide. .

If bad weather ruins your "Blue Moon" experience this weekend, don't worry. The online Slooh Community Observatory will offer a free live webcast of the May full moon beginning at 8 p.m. ET (0000 GMT). You can follow the Slooh webcast at Slooh.com

You can also watch the Blue Moon webcast on Space.com here, courtesy of Slooh.

Editor's note: If you snap an amazing photo of May's Blue Moon full moon and would like to share with Space.com and our news partners for a possible story or image gallery, send images and comments in to managing editor Tariq Malik at spacephotos@space.com.