You can tell a Blood Moon is out when the sky turns red. There is something about it that causes monsters to swarm.
A Blood Moon is a common, randomly occurring event that takes place over the course of an entire night. It causes several hazards for its duration: Enemy spawns increase, even near your Home and NPCs (where enemies do not normally spawn), Zombies become able to open Doors, and Bunnies and Goldfish transform into dangerous enemies, along with other effects (see below). Blood Moons are signaled by the message "The Blood Moon is rising..." in the lower left corner of the screen, and the moon and sky appear red for their duration.
A Blood Moon will only occur if at least one player in the world has over 120 life, and only on nights when a moon is present (any night except a new moon). Given those criteria, a Blood Moon has a 1 in 9 chance (11.1%) of occurring. Without taking the current moon into consideration, there is an overall chance of approximately 1 in 10 (1/9 * 7/8 = 7/72, or 9.72%).
 Blood Moon effects
- The spawn rate of monsters on the surface increases, and The Groom has a chance to spawn. Enemies not only spawn faster, but more are spawned than can normally exist on-screen at once. This works no matter the environment, so Corruption enemies can also swarm even more than they normally would, for example.
- Zombies become able to open a door after hitting them ten times. This can put the player's home and NPCs in peril if they are not guarded appropriately.
- Enemies can spawn even near NPCs and Home(s), whereas those usually suppress enemy spawns.
- All Bunnies and Goldfish become Corrupt Bunnies and Corrupt Goldfish.
Additionally, in hardmode:
 Other effects
- The store Inventory of most NPCs change:
- Deathweed will bloom. Blood moons are the only time you can harvest Deathweed Seeds from Deathweed.
- The Blood Moon's effects on monster spawn rates stack with that of the Water Candle and Battle Potion.
- Whether or not a Blood Moon will occur is determined at dusk each day.
- Every nightfall has an equal chance of being a Blood Moon, and does not change if the previous night was a Blood Moon. It is possible to have two or more Blood Moons in a row and, conversely, a Blood Moon will not necessarily occur even over the course of many in-game weeks.
- In a multiplayer game, using the time adjustment server commands (dawn, noon, dusk, midnight) do not trigger the Blood Moon check, and so using "dusk" repeatedly does not force a Blood Moon to occur.
- Newer players may want to barricade themselves indoors during the event. Doors can be barricaded easily by placing blocks of any type directly next to them. Other placeable items like the Table or a Torch will prevent doors from swinging inwards, which is the only way Zombies and Possessed Armor may enter. Placing all doors at least two tiles above ground, or with a 2x2 trench in front of them, will also prevent intruders. The player can also place two doors next to each other to prevent the intruders from entering.
- Blood Moons can benefit a fairly well-equipped player by providing quick money and items through enemy drops.
- With a Flail, open a door, throw the flail, and quickly close the door so the flail is stuck outside. Any approaching enemies will be hurt/killed on contact.
- More advanced players may want to use Mechanisms to construct lock-down systems to barricade entrances, and traps such as Lava "moats", to deal with Blood Moon and Goblin Invasion enemies. Active Stone Blocks can make simple or elaborate systems possible.
- With a Fish Statue or Bunny Statue and a 1 Second Timer, Corrupt Goldfish or Corrupt Bunnies will continuously spawn. This stops other enemies from spawning, and the player is able to make money by killing those relatively easy enemies. Corrupt Bunnies also provide the Bunny Hood, which sells for 40 silver.
- Using a Bunny Statue or a Fish Statue during a blood moon while in the underground hallow or corruption is a great way to quickly gather large amounts of Soul of Night and Soul of Light.
- Being near surface Hallow or Corruption means large spawns of those types of enemies during Blood Moons. This is useful for farming Mimics, since they drop so many rare items. This method also produces very large quantities of Pixie Dust, which is used in Greater Healing Potions.
- Female NPCs become unusually annoyed during Blood Moons (the Nurse and the Mechanic especially). This is most likely a crude joke based on "blood" and "that time of the month."