Rare Blue Moon Peaks May 31 — A Micromoon That Won't Return Until 2028

This May is bookended by two full moons, creating a calendrical blue moon on May 31. Despite its name, a blue moon isn’t blue in color; the term most commonly refers to a second full moon in a single calendar month or, seasonally, the third of four full moons in an astronomical season.

According to the Old Farmer’s Almanac, a seasonal blue moon occurs when an astronomical season (marked by solstices and equinoxes) contains four full moons and the third of those is labeled a blue moon. The popular modern meaning of the phrase became widespread after a mistaken definition published in Sky & Telescope in 1946 was amplified in the 1980s.

This year’s blue moon is also a micromoon — the smallest full moon of 2026. Astronomer Gianluca Masi, founder of the Virtual Telescope Project, told the Associated Press that Earth’s satellite will appear roughly 6% smaller and about 10% dimmer than an average full moon. Those differences are real but subtle, and most observers will not notice them with the unaided eye.

While the phrase "once in a blue moon" suggests extreme rarity, a blue moon averages about once every 2.5 years. The last monthly blue moon occurred on August 19, 2024, when it coincided with a supermoon, according to USA Today. Skywatchers hoping to catch this weekend’s spectacle should look skyward on the night of May 31, ideally from a dark location away from city lights; binoculars or a small telescope will reveal more lunar detail.

Sources: Old Farmer’s Almanac, Virtual Telescope Project, Associated Press (Marcia Dunn), USA Today (Julia Gomez), originally reported by Men's Journal (May 29, 2026).