The blue moon is May’s full moon. In 2026 it is full on Sunday 31 May 2026 at 9:45am BST.
Where the name comes from
Royal Museums Greenwich: A blue moon is not one of the twelve traditional names. The common modern definition is the second full moon in a single calendar month; the older, seasonal definition is the third full moon in a season that holds four.
Their guide to the full moon names is where that wording comes from, read on 10 September 2026.
Blue Moon is a name for a month, then, not for a kind of moon.
What there is to see
It is not blue. It is the same Moon, on a night that happens to be the second full moon of the month. The value of it is the arithmetic: a lunar cycle is about 29.5 days and most months are longer, so occasionally one month catches two.
The second full moon of May sits in the same low summer path as the first, thirty days later, at the very end of a month that has run out of darkness. Late May in the UK gives astronomical twilight rather than proper night. None of that stops you seeing it: a full moon is bright enough to read by, and the shortest, palest nights of the year are the ones it dominates most completely.
What people get wrong about it
That it is rare enough to justify the phrase. Once in a blue moon works out at roughly once every two and a half years, which is less often than a birthday and more often than a leap day. The seasonal definition, the older one, is rarer, and the two definitions disagree about which moons qualify.
A note on searching for it: the plain phrase “blue moon” mostly returns the 2026 film of the same name rather than anything astronomical. Adding the year is the quickest way past that.
Blue Moon dates
- 2026: Sunday 31 May 2026 at 9:45am (BST)
Times London local, from NASA and checked against the Royal Observatory. How these dates were checked.
Where this sits in the year
The blue moon is the sixth of the 13 full moons in 2026. The one before it is the Flower Moon on Friday 1 May 2026, and the one after is the Strawberry Moon on Tuesday 30 June 2026.
Full moon number 6 of 13 sits in the calendar with every date and London time. For what the Moon is doing on the nights either side of Sunday 31 May 2026 — waxing gibbous before, waning gibbous after — there is the eight phases of the Moon.
Why the date moves every year
Twelve lunar cycles run about eleven days short of a calendar year, so these names drift earlier until an extra full moon is squeezed in. That is exactly what the blue moon is.
Our bias, stated: we sell moon and lunar wall art. None of it is a blue moon as such, because there is nothing to draw — the blue moon looks like every other full moon. What can be got wrong is the drawing itself, and blue moon or no blue moon, how to tell an accurate moon phase print is the checklist for that.
Written by Craig Fearn, Aphelion Prints. Last updated .
