Apocalypse Weekend expands the reaches of Paradise with new maps and missions, set on Saturday and Sunday, adds new weapons and foes, and raises Postal 2's gore and violence to an even greater level. All normal cats are also replaced with "dervish cats", which spin in a manner similar to that of Looney Tunes Tasmanian Devil, attacking any nearby character when agitated. Dervish cats can also be collected and, in addition to muffling guns, can be thrown at NPCs to attack them. While gameplay is similar to its parent Postal 2, Apocalypse Weekend is not as open-ended. The gameplay is more linear in design, with the player mostly forced to follow a certain path to complete the game—typical of most first-person shooter games. In addition, the player cannot play as a pacifist and is forced to kill animals and zombies in order to progress in the game.
It can be downloaded or bought as part of Postal 2: Fudge Pack
Source: Wikipedia, "Postal 2", available under the CC-BY-SA License.