Every ~2 hours 48 minutes, or every ~7 days in game time
A Blood Moon is an event from Breath of the Wild.
Any night after Link has completed The Isolated Plateau and left the Great Plateau for the first time, the moon may turn red around 10:00 pm on the in-game clock. At around 11:30 pm, Malice speckles the air. When the time then hits midnight, the sky will turn red, and a cutscene will play with monsters appearing in a puff of smoke - the first time, Zelda's voice will explain:
"Link... Link... Be on your guard. Ganon's power grows...it rises to its peak under the hour of the blood moon. By its glow, the aimless spirits of monsters that were slain in the name of the light return to flesh. Link...please be careful."
All defeated enemies will then revive as if they were never killed, with the exception of bosses of quests marked as complete - principally, the four Blight Ganons and Master Kohga.
On subsequent Blood Moons, a shorter version of the same cutscene will play, and Zelda will simply say "The blood moon rises once again. Please be careful, Link..."
The main useful effect from a Blood Moon is that cooking meals while malice specks are in the air, from 23:30 on, during a blood moon will result in an automatic critical, improving the results markedly. The Shrine Quest Under a Red Moon also requires a blood moon to complete.
The character Hino, from Dueling Peaks Stable, keeps track of blood moons and can explain them to Link. In the day leading up to a blood moon, if Link asks him about the moon, he will reply "I'm getting that feeling again... Something is going to happen tonight. I can taste it!". During the blood moon, he will then become over-excited, running around the stable, and, if Link talks to him, his text will be written in red. He will calm down shortly afterward.
Blood moons regularly occur every 2 hours and 48 minutes of gameplay (every 7 days in-game). A blood moon can be delayed by entering a shrine or Hyrule Castle before the cutscene triggers, but the game will continue to trigger a blood moon every night thereafter until one can finally occur. If delayed too long, a so-called Panic Blood Moon may happen, where the system's free memory will fall below 10%, triggering a blood moon regardless of the in-game time to reset the game world and thus free system memory. If there's enough overload, there are instances where the Blood Moon may occur even in the middle of the day.[BM 1]
There are several exceptions to the Blood Moon depending on the location and task at hand[BM 2]:
- The blood moon may appear during the One-Hit Obliterator Trial in the Great Plateau, but it won't trigger a cut scene and will turn into a regular moon inconsequentially, meaning no malice wisps, no reddening of the sky and no storm clouds.
- On the lower sections of Hyrule Castle, there will be a red moon, the blood-red sky, the rolling clouds and more malice particles floating around, but won't trigger a cut scene either.
- Atop the spire of Hyrule Castle, the blood moon will yellow the sky and appear dark with rolling clouds, but won't trigger the cut scene.
- During the fight with Dark Beast Ganon and if it happens while reaching the Divine Beasts except Vah Rudania, the Blood Moon will turn to normal inconsequentially and with no weather signs. In Vah Rudania's case, it won't revive any Sentries if one is going up Death Mountain.
- While aboard the Divine Beasts, the blood moon does not appear, and as such, does not reset anything inside those.
- The Blood Moon doesn't happen during some mini-games like the Bird-Man Research Study and it fades even before the clock reaches midnight.
- In Master Mode, killed Sky Octoroks can respawn without a blood moon.
- If an enemy is killed in between 11:35 pm and 12:00 am on the in-game clock, monsters will not respawn.
- While it is known Blood Moons can be used to free system memory by resetting the game world, contrary to popular belief resetting enemy kill flags does not restore any memory.
First Blood Moon cutscene
Recurring Blood Moon cutscene
Example of a Panic Blood Moon during in-game daytime