Moon phase

The moon moves through different phases, indicated by the amount of it that is visible during the night. The phase of the moon affects which items some NPCs sell. The different phases include new moon (where no moon is visible) and full moon (where the entire moon is visible). In addition, the Blood Moon can be considered a phase.

New moon
A new moon is occuring when enemies spawn on ground level while it is night time and the moon is absent.

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

Full moon
A full moon is occuring when the entire moon is visible (full circle). The first night of a newly created world is always a 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.