When we say something happens "once in a blue moon," we mean it’s a rarity. This weekend, the phrase comes to life as a calendrical Blue Moon rises on Sunday, May 31.

But how did this colorful expression evolve from an ancient proverb into a modern astronomical event? The true story weaves together 16th-century satire, volcanic eruptions and a highly successful editorial accident.

From Impossible to Improbable

The phrase didn't originally mean "rare." It meant completely impossible.

In the 1500s, saying "the moon is blue" was the Renaissance equivalent of saying "when pigs fly." It was a phrase used to mock someone who would believe an obvious absurdity. In fact, a 1528 anti-clerical pamphlet declared:

"If they say the moon is blue, we must believe that it is true."

For centuries, the moon simply wasn't blue, and that was that. However, language is fluid. By the early 1800s, the idiom began to shift. The first recorded use of "once in a blue moon" to describe a real (although incredibly rare) event popped up in 1821.

The first recorded use of "blue moon" meaning a long period of time appeared in 1821 in a satirical book about British high society called "Real Life in London" which is attributed to writer Pierce Egan. The book describes two fashionably dressed characters bumping into each other on a London street. One excitedly shouts to the other:

"Why, Bill, how am you, my hearty? ... how's Harry and Ben? — haven't seen you this blue moon."

Because this was fresh London street slang at the time, the author assumed regular readers wouldn't understand it. He actually included a literal footnote at the bottom of the page to explain the slang, writing:

*Blue Moon—This is usually intended to imply a long time.

A few years later, in 1833, the phrase finally evolved into the exact wording we use today. It appeared in the British literary magazine The Athenæum during a review of a new stage opera, with the paper's critic grumbling that while foreign operas shouldn't completely crowd out British composers, "once in a blue moon such a thing may be allowed." Well then!

When Blue Moons Were Actually Blue

While the idiom was changing, in the second half of the 19th century, the moon actually looked blue.

The most famous instance occurred after the catastrophic 1883 eruption of Krakatoa in Indonesia. The volcanic explosion hurled immense plumes of ash and fine dust into the upper atmosphere. Because these dust particles were precisely the right size (about 1 micron wide), they triggered a physical phenomenon called Mie scattering.

The dust scattered away the long wavelengths of red light while allowing the shorter blue wavelengths to pass through. For nearly two years, people across the globe looked up to see a distinctly blue-tinted moon. Similar rare effects have been caused by massive forest fires and other major volcanic events throughout history.

The Accidental Modern Definition

Ironically, the astronomical "blue moon" we talk about today, which refers to two full moons in a single calendar month, was actually born from a mistake.

Historically, older almanacs (like the Maine Farmers' Almanac) used a "seasonal" definition: a blue moon was the third full moon in a season that contained four.

However, in 1946, Sky & Telescope magazine published an article that misinterpreted this complex rule, erroneously stating that a blue moon was simply the second full moon in a single calendar month. The mistake was catchy, easy to understand, and eventually cemented into global pop culture by the board game Trivial Pursuit in the 1980s. Today, astronomers accept both definitions.

This Weekend's Celestial Show: May 31, 2026

This weekend, we get to witness a calendrical blue moon with a fascinating structural twist.

Because the lunar cycle lasts 29.5 days—slightly shorter than our calendar months—the moon is managing to be full twice this month. Following the "Flower Moon" on May 1, the second full moon peaks at 4:45 a.m. ET on Sunday, May 31, 2026.

According to NASA and astronomical data, this event comes with a few unique features:

• The Micromoon Twist: This blue moon coincides with a "micromoon," meaning it will be at apogee (the farthest point from Earth in its orbit). It may appear up to 14% smaller and 30% dimmer than a supermoon, though the difference is subtle to the untrained eye.
• A Planetary Bonus: If you step outside to view it an hour after sunset on Saturday night, May 30, look west to spot Venus and Jupiter shining brightly. If you are an early riser an hour before daybreak on May 31, look east to catch Mars and Saturn.
• The Next Countdown: If you miss this weekend's show, you'll have to wait a while. The next seasonal blue moon arrives in May 2027, and the next calendrical blue moon won't grace our skies until 2028.

While the moon won't actually be glowing blue this weekend, stepping outside to view it connects us to centuries of folklore, atmospheric science and our ongoing obsession with the clockwork of the cosmos. And here's a hot tip: To the human eye, the moon will look just as full on Saturday, so take a glance then as well. And to make sure that you know exactly when to expect moonrise and moonset, make sure to check out the Sun & Moon page on The Weather Channel app.