Halloween

What do you use Halloween items for? I have them but not sure how to use them.

You need them to finish the Pumpkin achievements, he's located lower left on the screen, right above your pet. You also need them to craft pets. You have achievements that you're able to get different items...fall pets, avatars, stamps, diamonds, coins. The achievements are located by clicking on your avatar upper left corner on your screen.
Hope this answers your question.