Blood Moon

The  is an event in Terraria.

Overview
When night comes and it's a Blood Moon, a message saying "The Blood Moon is rising... " will appear in the bottom-left corner, and, rather than the regular night music, the "Eerie" song used in the Underworld and Meteorite will begin playing.

As the night progresses, the moon and sky will become progressively more red, an effect which peaks at midnight, and the water will be red. (added in 1.2)

The Conditions of a Blood Moon Rising
Each night has a one-in-nine chance of the Blood Moon rising when at least one of the players in the world has 120 or more health. The event can occur in pre-hardmode and hardmode.

Effects
During a Blood Moon, enemy spawn rates on the surface double, the ability of NPCs to reduce the spawn rate of nearby enemies is temporarily disabled, and the maximum number of enemies able to be spawned at once increases. It also becomes more difficult to keep NPCs safe, as Zombies gain the ability to open doors and Goldfish and Bunnies will turn corrupt. Do take note that the Blood Moon will not affect underground enemies.

Roughly 1 in 70 zombies spawned on a Blood Moon will be The Groom instead, and Doctor Bones becomes easier to find. If Hard Mode is activated, Blood Moons enable the spawning of Clowns and increase spawn rates of surface Hard Mode enemies as well.

Defensive
To stop Zombies from entering your home, dig a 2x2 trench in front of your door or doors. However, if you are in Hard Mode, you may have to worry about the Hard Mode monsters. Or if you're feeling kind of lucky, just dig a 4x4 hole in the ground and place lava, or collect Spikes from a Dungeon and place them on top of your house.

Dealing with the Blood Moon can be extremely hard, espicially if you're new to the game, or if you're in Hard Mode.

Another way to block out Zombies is to place a solid background object (ie. a Wood Platform or torches) on the inside of a door, so that the door cannot open inwards. This way, you can open the door to leave the house easily enough, but Zombies will not be able to open it no matter how much they bang on it (NPCs can still use the doors effectively).

If you have access to a Mechanic, then you can simply use Active/Inactive Stone Blocks as your door. However, this does not count as a door for NPC houses. But, if you make a house where each room has a door to the inside of the house, the Active Stone Block will serve as a wall, and the NPCs will stay. When it's inactive, if you leave it for too long, the NPC will leave.

However, the easiest and most effective way of blocking out zombies and other earth-bound monsters is to simply build your NPCs' houses suspended in the air (at least 10 blocks above the ground is recommended). If you do this, no further protection is necessary, as the enemies which can open doors or do not need doors cannot reach you, and the enemies which can reach you cannot open doors.

If you don't mind fighting and have early pre-hardmode armor or better it's always smart to be AWAY from your home to make it less likely for monsters to harm your NPCs. This is especially useful in hardmode because of the many types of monsters with a AI similar to the wraith because they can pass through walls.

Offensive
On the other hand, due to the increased rate of enemy spawning, fighting monsters on a Blood Moon is a great source of income, Lenses, and other materials. If you choose to fight on one of these nights, Silver Armor or better is recommended.

Players who do not want to fight, but want to collect drops from monsters on Blood Moons can do so by building a lava trap:
 * 1) Build a pit for the player to stand in and on top of.
 * 2) Place a roof that also is a pit 1 block thick.
 * 3) Fill it with 1 bucket of lava. Since monsters always try to reach the player, they will touch the lava and die.
 * 4) To test the pit, step in the lava. If damaged, the pit will function defensively.
 * 5) Throw an item in the pit. If the item is not destroyed, the pit is perfect.

Trivia

 * As of update 1.1, the Blood Moon has been given a more noticeable effect on friendly NPCs as well, primarily the female ones. Quotes from the Nurse, Dryad, Stylist and Mechanic all seem to show hostility or annoyance and even bad stress when acknowledging the player, compared to other days/nights. This is possibly a joke using "Blood Moon" as a reference to females' menstrual cycles.
 * The Blood Moon event may be based off of an real world event, known as a Lunar Eclipse. During the eclipse, the Moon can sometimes gain a reddish tint to it. This also goes hand-in-hand with both the real life and other Terraria event, known as a Solar Eclipse.
 * The Blood Moon also may be based on one of the "Seven Signs of the Apocalypse" from the Bible, in which it states that ,"...on the 6th sign the moon will arise, showing the color of blood, bringing with it the demons that prowled the Earth, but were sealed away. Then they will feast on those who doubted...".