This is a great question. The setting sets up what is about to come, the trial of Tom Robinson.
The first part of the book is playful. The children romp around throughout the summer, and they are fascinated with Boo Radley. They wonder about his life, and they want him to come out. To them, he is the bogeyman.
In chapter eight, things begin to change. Three events take place that serve as omens or prodigies of what is to come.
First, there is a freak snowstorm.
For reasons unfathomable to the most experienced prophets in Maycomb County, autumn turned to winter that year. We had two weeks of the coldest weather since 1885, Atticus said.
In literature, things that take place out of the ordinary usually signal that something ominous is about to take place.
Second, there is the death of Mrs. Radley. Here is what the text says:
Old Mrs. Radley died that winter, but her death caused hardly a ripple—the neighborhood seldom saw her, except when she watered her cannas.
All of this takes place within two paragraphs of chapter eight. The mood, therefore, changes. It is also in this chapter that Miss Maudie's house burns down, and the fire contributes to the change in the novel's mood; Scout and Jem's innocent summer is over, and the winter brings with it loss and the end of innocence. In the following chapters we see the ugly racism of Maycomb and the conviction of an innocent and good man.