Stargazers will get a rare lunar treat this weekend as a Blue Moon appears in the sky on May 31, 2026.
However, the Moon will not actually look blue. The term “Blue Moon” refers to timing, not color. In this case, it means the second full Moon in the same calendar month.
May has two full Moons this year. The first full Moon appeared on May 1, while the second will occur on May 31 at 4:45 a.m. ET. That makes the second one a calendar Blue Moon.
The last calendar Blue Moon was on August 30, 2023, while the next one is expected in December 2028.
A Blue Moon can be defined in two ways.
The modern calendar-based definition says a Blue Moon is the second full Moon within a single calendar month. This happens because the lunar cycle lasts about 29.5 days, while most months have 30 or 31 days.
As a result, if a full Moon happens on the first or second day of a month, another full Moon can occur near the end of the same month.
There is also a seasonal definition. A seasonal Blue Moon is the third full Moon in a season that has four full Moons. Under that definition, the May 31 full Moon does not qualify. The next seasonal Blue Moon is expected in May 2027.
No. The Moon will look like a normal full Moon.
The name does not mean the Moon will turn blue. A Moon can appear bluish only in rare atmospheric conditions, such as after major volcanic eruptions or large wildfires that release particles into the air.
This weekend’s Blue Moon is called “blue” only because it is the second full Moon of May.
The May 31 Blue Moon will also be a micromoon, meaning the Moon will be near the farthest point in its orbit from Earth.
EarthSky reported that this will be the most distant full Moon of 2026, at around 406,134 kilometers from Earth. That makes it appear slightly smaller than average, although most people will not notice the difference with the naked eye.
The full Moon will reach peak illumination at 8:45 UTC on May 31, which is 4:45 a.m. ET and 9:45 a.m. in the UK.
For many viewers, the best time to watch will be the night of May 30 or the early hours of May 31, depending on local moonrise and weather conditions.
Blue Moons are uncommon because most months have only one full Moon.
Since the lunar cycle is about 29.5 days, a second full Moon can fit into a month only when the first full Moon appears very early in that month. This is why a calendar Blue Moon happens roughly every two and a half years.
The May 31 event will therefore give skywatchers a rare chance to see a full Moon with a special calendar status, even if it does not look blue.
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