FEATURE: ‘Once-in-a-Blue-Moon’: Rare blue moon to light up the night sky on May 31

Tabuk City, Kalinga – Following the recent Blood Worm Moon that appeared on March 3, sky watchers across the Philippines and around the world are in for another celestial phenomenon on May 31, 2026, as the second full moon of the month—known as a Blue Moon—is set to rise in the evening sky.

The celestial event, which occurs roughly once every two to three years, offers moon gazers a unique opportunity to witness a full moon in all its brilliance, illuminating the night sky in spectacular fashion.

Although the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) has yet to issue an official confirmation of its visibility in the country, the Almanac reports that the Blue Moon will rise between 4:00 and 6:00 p.m. Philippine Standard Time. The moon will remain visible throughout the night until moonset in the early morning hours of June 1.

Should the moon be visible in the country, observers across the Cordillera region may enjoy excellent viewing conditions, as the area has many dark-sky locations where the moon should appear bright and crisp—especially in open spaces with minimal light pollution.

What to know about a Blue Moon

Despite its name, the moon will not actually appear blue in color. A Blue Moon, according to the National Aeronautics and Space Administration (NASA), refers to an extra full moon that occurs either within a season or within a calendar month—known respectively as a Seasonal Blue Moon or a Calendrical Blue Moon.

Typically, three full moons occur within a season, which is determined by the dates of the equinoxes and solstices. However, when a season contains four full moons, the third full moon is called a Seasonal Blue Moon.

Meanwhile, the second full moon within a single calendar month is referred to as a Calendrical Blue Moon. This happens because the moon takes about 29.5 days to complete a full cycle of phases. As a result, when a full moon occurs at the start of a month, another may appear toward the end of the same month—except in February.

The Blue Moon on May 31 is classified as a Calendrical Blue Moon—rare enough to be memorable, yet common enough to remind us why the phrase “once in a blue moon” continues to capture the imagination.

The last Calendrical Blue Moon occurred on August 31, 2023, while the last time two Blue Moons appeared in a single year was in 2018—during January and March. According to the Almanac, this phenomenon will not occur again until 2037.

Sky watchers are encouraged to find locations with minimal light pollution for the best viewing experience. While the moon will not appear blue in color, its rarity is what gives the event its name, making it a memorable sight for stargazers across the country.