Blue moons
The second full moon in a calendar month — 2 fall between 2025 and 2029. Nothing about it is blue, and it is a fact about the calendar rather than about the moon.
Next blue moon
December 31, 2028 · 16:48 UTC
The second full moon of that month — the first is on December 2, 2028.
Every blue moon, 2025–2029
first full moon that month: May 1, 202608:45 UTC
first full moon that month: December 2, 202816:48 UTC
Dates are UTC. Because a blue moon is defined by the calendar, a full moon in the first or last hours of a month can fall in a different month for you — see below.
Two definitions, and the mistake that created one of them
Monthly. The second full moon in one calendar month. This is what almost everyone means today, and it is what the list above uses.
Seasonal. The third full moon in an astronomical season that contains four rather than the usual three. This is the older rule, from the Maine Farmers' Almanac.
The monthly definition exists because of an error: a 1946 article in Sky & Telescope misread the almanac's seasonal rule as a monthly one, the simpler version spread, and by the time anyone corrected it the meaning had changed. Both are now in use.
This page deliberately does not list seasonal blue moons. Doing so needs the exact instants of the solstices and equinoxes, which this site does not solve for — and a list built on approximate season boundaries would be wrong in exactly the borderline cases that make it interesting.
Why your blue moon might not be a blue moon
A monthly blue moon is not an event in the sky — it is a coincidence between the moon's 29.5-day cycle and the length of a calendar month. That makes it dependent on which calendar, and which clock, you are reading.
A full moon at 00:20 UTC on the 1st has already happened on the 31st in the Americas. So a month can hold two full moons in London and only one in Los Angeles, or the reverse. Neither place is wrong, and there is no authority to appeal to — the moon did the same thing for both of them.
What is a blue moon?
Two different things, confusingly. The monthly blue moon — the one almost everyone means now — is the second full moon in a single calendar month. The seasonal blue moon, which is older, is the third full moon in a season that has four instead of the usual three. This page lists the monthly kind.
Is the moon actually blue?
No. The name has nothing to do with colour. A genuinely blue-looking moon does happen, very rarely, when smoke or volcanic dust of just the right particle size scatters red light — it was reported after the 1883 Krakatoa eruption — but it has no connection to the calendar event.
How often does one happen?
Roughly every two and a half years. It is a calendar artefact rather than an astronomical event: the moon's cycle is about 29.5 days and most months are longer, so occasionally two full moons fit inside one. In this 5-year range there are 2.
Why does the date depend on my time zone?
Because "which calendar month" does. A full moon at 00:30 UTC on the 1st is still the 31st in New York, which can move it into the previous month and create — or destroy — a blue moon depending on where you stand. The dates here are UTC, and a genuinely borderline case is called out on the page.
Where did the phrase come from?
"Once in a blue moon" is far older than either definition and simply meant something absurd or impossible. The modern monthly meaning traces to a 1946 magazine article that misread the older seasonal rule, and the mistake stuck.
More moon
Moon phase today · Moon calendar · Full moon calendar · Supermoons · Lunar eclipses