Blood Moon

The Blood Moon is an event that has a small chance (1/7) of happening every night that is not a new moon. They will only occur if at least one player on the world has 120+ hitpoints. It is heralded by the message "The Blood Moon is rising..." on the lower left side of the screen.

Blood Moon effects

 * The store Inventory of most NPCs change. The Merchant will sell Throwing Knives, the Arms Dealer sells Silver Bullets, the Clothier sells the Mime Mask, and the Dryad stops selling Purification Powder, Sunflowers, and Grass Seeds, and starts selling Vile Powder and Corrupt Seeds.
 * The spawn rate of monsters on the surface increases, and The Groom has a chance to spawn. The increase in the spawn rate includes not just the rate at which enemies spawn, but also multiplies the maximum number of enemies that can be on the screen at any given time (leading to sometimes impressive swarms). This works even when one is not in standard environments, so Corruption enemies can swarm even more than they normally would, as well.
 * Zombies gain the ability to open doors after punching them several times (10 for one zombie). This can put the player's home and NPCs in peril, if they are not guarded appropriately.
 * Enemies can spawn even when one is near NPCs and his/her Home(s). NPCs usually have a cumulative effect of suppressing enemy spawn rates, and tightly clustered NPCs can usually prevent the spawning of any enemies at all within the radius of a few screens. During the blood moon this effect ceases in addition to the increased spawn rate, causing what appears to be an even more dramatic upswing in the spawn rate.
 * All Bunnies and Goldfish will become corrupt.
 * Deathweed will bloom. Blood moons are the only time you can cut deathweed to cause deathweed seeds to drop.

Additionally, in hardmode:
 * Possessed Armor can open doors, similarly to zombies.
 * Clowns have a chance to spawn

Tips

 * The Blood Moon can benefit intermediate players by providing quick money and items.
 * 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 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.
 * Newer players who want to pick a fight may do well to fight from within their own homes. Small caves also make decent areas to make a stand. Fighting from inside of one's house or any other enclosed space prevents zombies from spawning on both sides of the player (if fighting on open terrain) or dropping down from above (if backed up to a cliff), reducing combat to the mere slog of swinging one's weapon of choice continuously until dawn breaks.
 * More advanced players may want to go beyond merely putting their doors out of zombie reach, and build lava "moats" to let zombies walk into. While unnecessary, and not even particularly profitable at the stage of the game where lava becomes available, many players find an endless stream of zombies walking into a lava pit and exploding en masse quite amusing.
 * It is possible to place any block in front of or behind a door, open the door (or let it be opened/broken), and then wisely kill all of the zombies that attempt to get in.
 * Using Wire, one can easily set up a full base auto lockdown system by placing Active Stone Blocks in front, or behind the doors, or as doors.
 * If a player has access to the Fish Statue, he could make a fishtank of some sorts, connect a 1 Second Timer and watch the corrupt goldfish spawn. This has 2 advantages, first and foremost, the massive amount of (corrupted) goldfish spawning in will stop the zombies and other mobs from spawning, and second, the player is able to make staggering amounts of money by killing all the corrupt goldfish when the blood moon ends. Bunny Statue will also work.