Before opening game, change local clock to be October 31st of whatever year so that the game thinks it is time for the event.
Create new game - make sure to put on the bloody clothes in character creation - shirt pants shoes, I also picked the eyes that have no pupils (not sure if that part matters or not). Pick musician (have tried with gamer and chef with no success) and start the game. Progress until you are into the apartment. Get enough money to buy a couple of the Halloween foods, I did 2 of the soup stuff. After that, purchase the pumpkin from the Idea store and as soon as you make the purchase you should get the achievement.