Once in a blue moon
When Is the Next Blue Moon?
The next blue moon — a month's second full moon — falls on Sunday, December 31, 2028.
Despite the name, a blue moon has nothing to do with colour. In the definition almost everyone uses today, a blue moon is the second full moon to fall within a single calendar month. It can happen because the lunar cycle runs 29.5 days — just shorter than every month except February — so a full moon landing on the 1st or 2nd leaves room for a second one before the month ends.
That squeeze is uncommon: it works out to about one blue moon every two to three years, which is exactly why "once in a blue moon" means rarely. February, always shorter than the cycle, can never host one — and in the same years February sometimes gets no full moon at all (a "black moon" month).
There is also an older, farmer's-almanac definition: the seasonal blue moon, the third full moon in an astronomical season that contains four instead of the usual three. The monthly version — popularised by a 1946 Sky & Telescope article that misread the almanac rule — is the one that stuck. Both are ordinary full moons; the sky looks no different.
Can the Moon ever look blue? Very rarely, yes — after major volcanic eruptions or forest fires, when airborne particles of just the right size scatter red light and let blue through. Observers reported blue-tinted moons after Krakatoa in 1883 and Canadian fires in 1950. It has nothing to do with the calendar kind.
Upcoming blue moons
| Date (UTC) | Month | Time (UTC) |
|---|---|---|
| Sun, December 31, 2028 | December | 18:29 |
| Tue, September 30, 2031 | September | 19:29 |
Monthly (calendar) blue moons, computed in UTC. In your local time zone a borderline full moon can shift a day either side of midnight, which occasionally moves a blue moon to a different month.
Frequently Asked
When is the next blue moon?
The next calendar blue moon — the second full moon in a single month — is Sunday, December 31, 2028 at 18:29 UTC.
What is a blue moon?
In the common modern definition, a blue moon is the second full moon that falls within one calendar month. The older, seasonal definition names the third full moon in a season that has four.
Is a blue moon actually blue?
No — it looks like any other full moon. The name has nothing to do with colour. Genuinely blue-tinted moons are extremely rare and caused by smoke or volcanic ash filtering red light.
How often does a blue moon happen?
Roughly every two to three years. The lunar cycle is 29.5 days, slightly shorter than most months, so occasionally two full moons squeeze into one calendar month.