Moon phase

The Moon in Terraria moves through a cycle of eight different phases, with the next consecutive phase appearing each night. The current moon phase is indicated by the amount of it that is visible each night. The phase of the moon affects which items certain NPCs sell, as well as certain enemy spawn rates. The first night of a newly created world is always a full moon.

It takes only eight in-game nights for the Moon to move through all of its phases. The different phases include the full moon, where the entire circular moon is visible; and four nights later, the new moon, where no moon is visible. Another four nights later, the full moon returns.

A Blood Moon can occur during any moon phase except for a new moon.

New moon
During a new moon, Wraiths, Demon Eyes and Wandering Eyes spawn more often.

Full moon
During a full moon:
 * The Clothier will sell the Plumber's clothes.

If Hardmode is activated:
 * Werewolves will start to spawn, being as common as zombies.

During the day after a full moon:
 * The Clothier will sell The Doctor's clothes.

Blood Moon
When a Blood Moon occurs, the sky turns red. During a Blood Moon:
 * The Arms Dealer will sell Silver Bullets. (Pre-Hardmode)
 * The Clothier will sell the Mime Mask.
 * The Merchant will sell Throwing Knives.
 * The Dryad will stop selling Grass Seeds, Purification Powder and Sunflowers, and will start selling Corrupt Seeds and Vile Powder.
 * Bunnies and Goldfish will become corrupted.
 * Zombies and Possessed Armors start breaking down doors.
 * The Groom, who is a Zombie may appear and drops the Top Hat upon being killed.
 * During Hardmode the Clown can appear on the surface.
 * Spawn rate and max spawns are increased.