Final answer:
In the book 'Wonder,' Summer dresses as a unicorn for Halloween, marking a significant moment in the story.
Step-by-step explanation:
In the book Wonder, Summer, one of the main character's friends, ends up dressing as a unicorn for Halloween. This is contrary to previous years where characters have dressed around a common theme, but in this instance, Summer's choice is more individualistic and stands out due to its uniqueness. In Wonder, Halloween plays an essential role as it reveals the different personalities and quirks of the characters through their costume choices. It also becomes a pivotal moment in the story that affects the protagonist, Auggie.