Blue Moon — May 31, 2026 (Monthly)

What this countdown tracks

The clock counts down to the monthly Blue Moon of Sunday, May 31, 2026 — the second full moon to fall within the calendar month of May 2026, and the first calendar Blue Moon since August 2023.

About this celestial event

A Blue Moon is, in popular usage, the second full moon to occur within a single calendar month. The full moon cycle is 29.53 days long, so most calendar months contain only one full moon; once every roughly two and a half years, a month contains two. The first full moon of May 2026 falls on May 1; the second — the Blue Moon — falls on May 31.

The Blue Moon has a more technical second definition. The original meaning, used by the Maine Farmers' Almanac in the early 20th century, is the third full moon of an astronomical season that contains four full moons (rather than the usual three). The "second-full-moon-in-a-month" definition is a more recent reinterpretation — it dates from a 1946 Sky & Telescope article that misread the almanac convention. Both definitions are now in common use; this Blue Moon is the calendar (monthly) variety.

The Moon will not actually look blue. The "Blue Moon" name predates either definition and refers to the rare appearance of an actually blue-tinted moon caused by atmospheric scattering after major volcanic eruptions or wildfire smoke (the eruption of Krakatoa in 1883 produced famously blue-coloured moons for years afterward). The colour is unrelated to the calendar event.

What to expect

The May 31, 2026 Blue Moon reaches full phase visible all night from rise to set. The Moon will look entirely normal — silver-grey, possibly with a yellow or orange cast near the horizon — and will be near apogee, so it will appear slightly smaller than an average full moon.

Best viewing is at moonrise on the evening of May 31 from an open eastern horizon, and through the night with the Moon high in the south. Because Blue Moons are so frequently invoked in popular culture ("once in a blue moon"), they have a strong appeal for casual observers. Major observatories and astronomy clubs run Blue Moon evenings.

Past blue moons

• August 31, 2023 — Blue Moon (and supermoon) — the most-photographed Blue Moon of recent years
• October 31, 2020 — Halloween Blue Moon, first since 1944
• March 31, 2018 — second Blue Moon of 2018, an extremely rare double-Blue-Moon year
• January 31, 2018 — Super Blue Blood Moon (Blue Moon, supermoon, lunar eclipse)
• July 31, 2015 — Blue Moon
• August 21, 2013 — seasonal Blue Moon (third full moon in a four-full-moon summer)

How to observe

NASA's Daily Moon Guide, EarthSky, Sky & Telescope and timeanddate.com publish moonrise/set times for thousands of cities. Stellarium and SkySafari give precise local timings. The Virtual Telescope Project streams Blue Moon events over Rome. No equipment is needed; the Blue Moon is enjoyed naked-eye.

Related countdowns

The May 2026 Blue Moon precedes the seasonal Blue moon May 2027 by exactly one year. Pair with the Eta Aquariid meteor shower 2026 and June solstice 2026 on the late-spring sky calendar.

FAQ

When is the May 2026 Blue Moon? Sunday, May 31, 2026 — the second full moon of May 2026. Will the Moon actually be blue? No. The "Blue Moon" name is a calendar quirk, not a colour description. How often do Blue Moons happen? Roughly every 2.5 years on average for the calendar definition; less frequently for the seasonal definition. What's the difference between a calendar Blue Moon and a seasonal Blue Moon? Calendar = second full moon in a calendar month. Seasonal = third full moon of an astronomical season that contains four. The next seasonal Blue Moon is May 20, 2027.