The Pumpkin Moon Medallion is an item added in v1.2.1 used to summon the Pumpkin Moon, which must be used during the night. If used during the day, it will not be consumed, and the Pumpkin Moon will not be summoned.
This item can only be obtained legitimately in Hard Mode, due to the fact that it is crafted using Ectoplasm and Hallowed Bars, unless the ingredients are brought from a different world.
Using the Pumpkin Moon Medallion on a Blood Moon will cause the Blood Moon to be replaced by the Pumpkin Moon.
The Pumpkin Moon is similar to the Solar Eclipse and Blood Moon events, therefore dying will not end the Pumpkin Moon.
The Pumpkin Moon Medallion can be made and used after the Halloween period.