A “Once in a Blue Moon” event occurs on May 31, 2026, when the second Full Moon of the month rises—commonly known as a Blue Moon. The Moon will not actually be the color blue, so don’t be disappointed. So, what is a Blue Moon, exactly? Read all about the different types of Blue Moons and tell us what you think.
- Is “Blue Moon” a scientific term used in astronomy?
- Did this term originate with Native American folklore, like a number of the other Full Moon names?
- Does a Blue Moon really look blue?
In truth, the answer to all of these questions is “no.”
The modern understanding of “Blue Moon” only took off in the 1980s. It was a result of a much earlier mistake printed in a 1946 issue of Sky & Telescope magazine, and since then, the term has gone viral in the media.
Next Blue Moon: May 31, 2026
The next Blue Moon is on Sunday, May 31. Technically, it crests at 3:45 A.M., near moonset.
Two Types of Blue Moons
There are two definitions of the term, though we commonly use the second term (a second Full Moon in one month).
Seasonal Blue Moon
The extra Full Moon that occurs within an astronomical season. One season—defined by the dates of the solstices and equinoxes—typically has three Full Moons occurring within it. If a season instead has four Full Moons, then the third Full Moon (not the fourth) in the season may be called a Blue Moon.
Calendrical Blue Moon
The second Full Moon to occur in a calendar month. It takes our Moon about 29.5 days to complete one cycle of phases (from New Moon to New Moon), so if a Full Moon occurs on the first of a month, there will be a second Full Moon—a Blue Moon—at the end of the month, too (except in February).
Although the latter definition is the one more commonly followed today, the former actually came first. As mentioned above, a misinterpretation of the seasonal definition in the 1940s led to the calendrical definition, which was later popularized in the 80s and has persisted to today.
How Often Does a Blue Moon Occur?
Most months have one Full Moon, not two.
Since the Moon’s period of phases is 29 1/2 days, while months usually have 30 or 31 days, it’s obvious that if a Full Moon lands on the first day of any month except February, it will repeat again at the end.
Turns out, calendrical Blue Moons happen every 30 months on average. Two and a half years. Seasonal Blue Moons happen at a similar rate: about once every two to three years. So maybe “Once in a Blue Moon” isn’t so rare after all!
Can There Be Two Blue Moons in One Year?
How often do we have two Blue Moons in a single year? (As in 2018, when there were Blue Moons in both January and March and no Full Moon in February.)
This won’t happen again until 2037, when we’ll have another Blue Moon in both January and March.
Learn More About The Moon
Want to learn more about the Moon? See the Almanac’s Moon Phase Calendar and Full Moon Calendar!
The Blue Moon on May 31, 2026, is a calendrical Blue Moon—rare enough to be memorable, but common enough to remind us why the phrase “Once in a Blue Moon” still captures our imagination.
Source: , The Old Farmer’s Almanac
Photo Credit: space.com | A Blue Moon is a rare occurrence that depends on the timings of full moons during the year. (Image credit: Colin Wooderson / 500px via Getty Images)
