Blood Moon

The Blood Moon occurs on seemingly random nights in.

Overview
The Blood Moon is indicated by a red-colored full moon rising instead of a regular moon or phase, starting after Link leaves the Great Plateau. At around 11:30 PM, the environment will glow red and the air will be full of black particles. When it occurs, the Blood Moon revives all enemies and Overworld Bosses that have been defeated up until that point. This includes Guardians, including the ones encountered in an Ancient Shrine, allowing Link to retake the Tests Of Strength combat trials whenever he pleases, though he cannot obtain more Spirit Orbs from doing so, as the trial will remain cleared and the monk will be absent.

According to Princess Zelda, the Blood Moon is an effect of Ganon's power reaching a peak. Thus, it can be safely inferred that it is Ganon himself who is reviving the monsters, as it is only at this time that it occurs.

A Blood Moon event can also occur even when skipping time using a Campfire, Cooking Pot, or by sleeping at an inn, as its cutscene still plays after time has been skipped. If a Blood Moon occurs when Link is in an Ancient Shrine or Hyrule Castle, the Blood Moon scene does not appear and previously defeated enemies will not be revived. The Blood Moon will occur every following night until it happens while Link is outside where the scene can appear.

Cooking from 11:30 PM to 12:15 AM on the night of a Blood Moon gives Food an effect boost.

Trivia

 * An odd exception to the Blood Moon failing to trigger at midnight while in Hyrule Castle is during the first fight against Calamity Ganon. If midnight strikes on a Blood Moon night during this first battle, Calamity Ganon's health will refill to maximum as if neither Link nor the Divine Beasts had done him any damage. Strangely, this exception does NOT apply to the second battle against Dark Beast Ganon.
 * It has not been confirmed whether the Blights are healed or left alone if fought on a Blood Moon night.
 * In addition to reviving enemies and Overworld Bosses, ore deposits are replenished, Treasure Chests are refilled, and Materials ranging from plants and trees, to gear left lying in the field, effectively respawn, allowing Link to collect them again if the ones he took before the Blood Moon broke in the interim period.
 * From a meta perspective, the Blood Moon exists to reset the game world, allowing the player to always be able to find materials no matter how long they play. So defeating enemies and Overworld Bosses, and otherwise picking the land clean of resources, increases the likelihood of a Blood Moon occurring the next night.
 * Hino of the Dueling Peaks Stable is the primary source of information on the Blood Moon, and can be seen acting odd during the time period that it affects the overworld. After midnight, should Link be curious if he can still reap the benefits of cooking under a Blood Moon, he need only see if Hino is still acting odd. If he is normal again, the effect has expired.