This has been covered in detail elsewhere (check out the themes of "Good vs. Evil" and "Time" for more), but we wanted to remind you again of this powerful motif in the novel. In Chapter 38, Mr. Halloway defines the carnival people as autumn people, or, in other words, Evil. Based on newspaper research, he learns that the carnival comes to Green Town only in October. In some of the most beautiful language in the novel, he talks about how the carnival is made up of people who live in perpetual autumn, or, in other words, reside in and feed off of evil.
Presented in opposition to autumn is summer. (Which is why you should check out Ray Bradbury's Dandelion Wine, because it covers summer in Green Town.) Mr. Halloway, our resident guru on good vs. evil, tells Will and Jim that they are sometimes summer people and sometimes autumn people. Both tendencies, the good and the bad, live within us.