Implemented since October 31, 2015 in the following release of the fourth game's Halloween Edition update, this update can occur once every year when the date hits October 31st. Although, the player can actually get to see this whole change any time by modifying the date to October 31st on their computer.
When on the said date, the player's Office will be automatically decorated with Halloween lights and a Jack-o-lantern for a little fun Halloween celebration. Other than being used as environmental props, they serve no actual gameplay functions.
The Jack-o-lantern seen in the first Five Nights at Freddy's game is a same pumpkin accompanied with Jack-O-Chica, albeit slightly re-textured and added with bulbs for eyes.
The Jack-o-lantern used for Five Nights at Freddy's.
The Jack-o-lantern used for Five Nights at Freddy's 2.
The Jack-o-lantern used for Five Nights at Freddy's 3.