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 not occur unless at least one player has more than 6 hearts. It is heralded by the message "The Blood Moon is rising...".

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), and this works even when you are 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 you are near NPCs and your Home(s). NPCs usually have a cumulative effect of suppressing enemy spawn rates (and tightly clustered NPCs can prevent the spawning of any enemies at all within a 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.

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 each, will also prevent intruders.
 * Newer players who want to pick a fight may do well to fight from within their own homes, anyway. Small caves also make decent areas to make a stand. Fighting from inside your house or other enclosed space prevents zombies from spawning on both sides of you (if you fight in open terrain) or dropping down on you from above (if you simply backed up to a cliff), reducing combat to the mere slog of swinging your 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.