Trick or Treat

Trick or Treat, originally called Angry Birds Halloween, is the first holiday-themed version of Angry Birds and Halloween-themed episode. It was released on October 21, 2010. It is based on the holiday of Halloween, and as such, features a dark, spooky environment. Pumpkins and jack-o'-lanterns appear as obstacles, the latter capable of giving 3000 points if destroyed, and the Pigs wear small pumpkin hats and sometimes are hidden inside a pumpkin. One Large Pig on Trick or Treat 3-15 appears wearing a ghost costume, swinging on a suspended platform.

Storyline
The following is from The World of Angry Birds Official Guide, as told by the Mighty Eagle:

NIGHT OF THE EGG SNATCHERS I like dark and stormy nights. They remind me of my cave. But none was as terrifying as the Halloween when the birds found their nest empty. Had the eggs been grabbed by gruesome zombies, fearsome vampires or wicked witches? No, it was just the pigs — although Smooth Cheeks' face is just as freaky as any fright mask if you ask me!

Trivia

 * This is the only seasonal episode to have opening, middle and ending Cutscenes. This is probably because this was the only planned episode at first.
 * On mid-2012, the update that included Piglantis, it was then removed and replaced by an ending photo, like all of the episodes in the game.
 * There is a comic strip based on this episode called Angry Birds: A Halloween Special.
 * This is the first episode to have a large pig instead of the helmet pig on the the first cutscene.
 * This is the first episode to remove their cutscenes and replaced with a newer ending photo.
 * Before the idea of Angry Birds Seasons the game Angry Birds Halloween, was originally going to feature Halloween themed episodes rather than levels themed on Seasons.
 * Bomb should have been behind the pumpkin seen in the cutscene where Red and Matilda are on the other side.