If you are playing Minecraft on Halloween day (alternatively you can go offline and set your system's date to Halloween) and you put the difficultt on easy, normal, or hard, mobs will have approximately a 1/5 chance of spawning with a pumpkin/jack-o-lantern on their head. This greatly increases the difficulty of survivng against hostile mobs in the game, since mobs who spawned with a helmet of any type(like a pumpkin) wont be harmed by sunlight. The mobs affected by this are typically zombies, skeletons, wither skeletons, baby zombies, zombie pigmen, and baby zombie pigmen.