You don't specify which summer you mean in the story. Dill's appearance in Chapter 1 provides both Jem and Scout with a new playmate, and Dill quickly becomes the final member of their inseparable triumvirat. Dill not only becomes the best friend of both of the children, but he also becomes Scout's love interest: They share kisses and Dill becomes Scout's "permanent fiance." Dill's more unexpected appearance in Chapter 14, when he runs away from home because of the supposed ill-treatment by "his new father, who disliked him," and shows up under Scout's bed, is thrilling news for Scout, since Dill was not expected to spend this summer in Maycomb.
I had never thought about it, but summer was Dill by the fishpool smoking string, Dill's eyes alive with complicated plans to make Boo Radley emerge... With him, life was routine; without him, life was unbearable. (Chapter 12)
After they spend an innocent night together sharing Scout's bed, it becomes the most exciting summer yet for Scout, since Dill joins the Finch children on all of their before-and-after trial experiences: They save Atticus and Tom from the lynch mob; Dill accompanies Jem and Scout to witness the rape trial; he is with Scout when Dolphus Raymond reveals his secret about the contents of his ever-present paper sack; and Dill recounts to Scout the moment when Atticus has to break the news about Tom's death to Helen Robinson.
Dill first visits Maycomb in Chapter 1 and quickly becomes close friends with Jem and Scout. Dill entertains both of the children and plays most of the character parts "thrust upon" Scout during their playtime dramas. Dill also comes up with elaborate plans to get Boo Radley to come out of his house. Dill's fascination with Boo Radley leads the Finch children on many adventures and brings excitement to their summer by putting them in some very precarious situations. Scout also becomes the object of Dill's affection before he begins ignoring her in order to spend more time with Jem.
When Dill runs away from home and returns to Maycomb in Chapter 15, the atmosphere of the summer is drastically different. Upon Dill's arrival, the Tom Robinson trial is in full swing, and each one of the children loses their childhood innocence after witnessing racial injustice firsthand. The carefree days of attempting to see Boo Radley are over, and the reality of Maycomb's overt prejudice takes center stage.