Pumpkins were introduced on October 25, 2013 as a part of the Halloween update. Pumpkins are used as currency in the Pumpkin Shop, which is accessed by the command /pshop. However, at the time of writing, prices for the /pshop have not been set and items cannot be bought.

Pumpkins can be earned by killing enemies, winning games, and opening care packages. Killing an enemy earns a specific amount of pumpkins depending on what the player is.

Source Pumpkins Earned
Killing as a mother zombie 3
Killing as a zombie 2
Killing as a human 1
Winning a round 3
Opening or defending a care package 10

If at least a certain number of players are on a server during the Halloween event, than when there are 110 seconds left in a round, fireworks will be continuously fired out of a random location. 15 seconds later, a care package will land on that spot. Humans can right click the care package to get a pumpkin pie, a pumpkin grenade, or 10 pumpkins. If zombies can prevent the care package from being opened by 5 different humans for 30 seconds, then zombies will get one of the same rewards or the use of the spider kit for the remaining 65 seconds. It is possible for a human to open the care package, die, and then get the reward for defending it from other humans, thus being rewarded twice.