The second full moon in a calendar month — and the reason "once in a blue moon" means almost never. What it really is, why the Moon isn't actually blue, and every blue moon through 2028.
| Date | Type | Note |
|---|---|---|
| May 31, 2026 | Monthly | 2nd full moon in May (1st was May 1) |
| Dec 31, 2028 | Monthly | 2nd full moon in December — a New Year's Eve blue moon |
| Aug 21, 2029 | Seasonal | 3rd of 4 full moons in one astronomical season |
| Sep 30, 2031 | Monthly | 2nd full moon in September |
There are two definitions. The monthly blue moon — the second full moon in a calendar month — is the one everyone uses today. The seasonal blue moon — the third full moon in an astronomical season that contains four instead of the usual three — is the older, original meaning from the Maine Farmers' Almanac. The monthly version became dominant after a 1946 Sky & Telescope article accidentally simplified the seasonal rule; the mistake stuck and is now the popular standard.
The colour is a red herring. The most accepted origin traces "blue" to the old English "belewe", meaning "to betray" — a "betrayer moon" was an extra full moon that disrupted the calendar of religious fasts like Lent. Over centuries "belewe moon" drifted into "blue moon". The Moon can rarely look genuinely blue when wildfire smoke or volcanic ash (famously after Krakatoa in 1883) fills the air with particles of just the right size to scatter red light — but that optical effect has nothing to do with the calendar blue moon.