What Is a Blue Moon?

Quick Answer

A Blue Moon is an extra full moon, most often the second full moon in a calendar month. In 2026, it falls on Sunday, May 31, 2026, with the moon in Sagittarius. The name originates from the Maine Farmers’ Almanac’s seasonal rule, with the second-full-moon-in-a-month meaning arising from a 1946 magazine article.

Date: Sunday, May 31, 2026 · Zodiac: Sagittarius · Also called: Monthly Blue Moon, Seasonal Blue Moon · Next full moon: Strawberry Moon (June 29, 2026)

What Is a Blue Moon?

A Blue Moon is a full moon that is considered an extra one within a calendar unit. It is a matter of timing rather than appearance: the Moon looks the same as any other full moon.

The Moon takes about 29.5 days to complete a cycle of phases, which is slightly shorter than most calendar months. Over time, the extra days accumulate, and occasionally a month holds two full moons or a season holds four.

What Are the Two Definitions of a Blue Moon?

Two definitions of a Blue Moon are in use. The monthly Blue Moon is the second full moon in a single calendar month. This is the definition most widely used today.

The seasonal Blue Moon is the third full moon in an astronomical season that contains four, with seasons running from solstice to equinox and equinox to solstice. This is the original definition. It appeared in the Maine Farmers' Almanac, which used it to keep the traditional names of full moons aligned with the seasons.

The monthly definition became popular after a 1946 magazine article misread the almanac's rule, and it spread through radio and later publications. Both definitions are now accepted.

Why Is It Called a Blue Moon?

The phrase 'once in a blue moon' predates either astronomical definition and has long meant something that rarely happens. The Moon itself is not usually blue, so the saying originally described the improbable.

The Moon can appear blue in rare atmospheric conditions. Volcanic ash, wildfire smoke, and fine dust can scatter red light and let blue light pass through. After the eruption of Krakatoa in 1883, blue and even green moons were reported around the world. Similar effects followed the 1950 forest fires in Canada and the 1980 eruption of Mount St. Helens. These events are unrelated to the calendar definition.

When Is the Next Blue Moon in 2026?

The 2026 Blue Moon falls on Sunday, May 31, 2026, with the Moon in the zodiac sign of Sagittarius. It is the second full moon of May 2026, following the Flower Moon on May 1, which makes it a monthly Blue Moon.

• Previous full moon: Flower Moon — Friday, May 1, 2026
• Blue Moon: Sunday, May 31, 2026 (Moon in Sagittarius)
• Next full moon: Strawberry Moon — Monday, June 29, 2026

How Often Do Blue Moons Occur?

A Blue Moon occurs roughly once every two to three years, on average about every 2.7 years. Because a full moon cycle is about 29.5 days and most months are 30 or 31 days, a second full moon in a month is only possible when the first falls in the first day or two. February cannot hold two full moons in most years.

Monthly Blue Moons are more common than the seasonal kind in the modern calendar. The two types occur in different years, and the time between them varies.

What Are Some Famous Blue Moons in History?

The best-documented blue-tinted moons followed volcanic and fire events rather than calendar quirks.

• Krakatoa, 1883 — the eruption sent ash high into the atmosphere, and blue and green moons were reported for years
• Canadian forest fires, 1950 — smoke from fires in northern Alberta produced blue moons seen across North America and Europe
• Mount St. Helens, 1980 — ash in the atmosphere led to reports of blue-tinted moons
• 1946 magazine article — the misreading that established the monthly definition now in common use

What Is the Spiritual and Astrological Meaning of a Blue Moon?

A Blue Moon is commonly regarded as a rare opportunity and a point of completion. Because it is an extra full moon, it is traditionally associated with finishing what was begun at the preceding full moon and reflecting on what has been learned.

Astrologically, the 2026 Blue Moon occurs in Sagittarius, a Fire sign ruled by Jupiter and associated with expansion, belief, learning, and travel. A full moon in Sagittarius is traditionally linked with themes of perspective and growth. Because the Sun is in Gemini at the same time, the Sagittarius–Gemini axis emphasizes the balance between broad vision and detail.