Blood Moons are signaled by the message 'The Blood Moon is rising.' in the lower left corner of the screen (or the top middle of the screen on Mobile).
It causes several hazards for its duration: Enemy spawns increase, even near the player's Home and NPCs (where enemies do not normally spawn), Zombies become able to open Doors, and Bunnies, Penguins, and Goldfish transform into dangerous enemies, along with other effects (see below). A Blood Moon is a common randomly occurring event that takes place over the course of an entire night however, a Blood Moon can also be initiated when the player uses a Bloody Tear.