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.

Arms Dealer
The Arms Dealer will sell Illegal Gun Parts during a new moon. He will sell Silver Bullets during a Blood moon and Unholy Arrows after Eater of Worlds is defeated.

Clothier
The Clothier will sell The Doctor's clothes during the night after a full moon, the Plumber's clothes during a full moon, and the Mime Mask during a blood moon.

Merchant
The Merchant will sell Throwing Knives during a Blood Moon.

Dryad
The Dryad will sell Corrupt Seeds and Vile Powder during a Blood Moon.