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:

Full moon
A full moon is occuring when the entire moon is visible (full circle). During a full moon:
 * The Clothier will sell the Plumber's clothes.

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

During a full moon Werewolves will spawn, they will be as common as zombies (confirmed).

Blood Moon
When a Blood Moon occurs, the sky turns red. During a Blood Moon:
 * The Arms Dealer will sell Silver Bullets.
 * The Clothier will sell the Mime Mask.
 * The Merchant will sell Throwing Knives.
 * The Dryad will sell Corrupt Seeds and Vile Powder.