People prepare for the night of ghosts and goblins in different ways. Some go trick-or-treating, while others get so drunk they start believing they are their costume. Only a select few lock themselves away to craft free Halloween updates for their games, and it turns out Terraria's developers happen belong in the latter group.
Terraria's “Halloween event” update fills your biome with Halloween-themed pets, costume paintings, gear, and friendly NPCs to sell you those items. Here's hoping they don't use your hard-earned cash to egg the impenetrable fortress you spent hours refurbishing. Slain monsters will also drop goodie bags containing “fun surprises” that hopefully won't include mini bottles of toothpaste or, ugh, pencils. People who give those away on Halloween are the true monsters.
There's also a “Pumpkin Moon Event” that can be triggered in hard mode. Developer Redigit was light on the details, but made the thing sound like a survival mode where you stave off waves of enemies. The more waves you survive, the better the spoils. The Halloween event lasts until Nov. 10, probably to let you recover from whatever candy coma you fall into.
There are also a bunch of non-Halloween themed bug fixes and balances. I found the most adorable one was “Mice can no longer spawn in hell.” That's just my personal pick. Feel free to check out the full list and make your own decisions. Or, you could just tinker around while munching on your sugary-dessert of choice—preferably in a soundproof chamber where no doorbell can reach you.