Something Wicked this Way Comes was written by Ray Bradbury in 1962. The year the story takes place is not actually specified. All we know is that it opens on October 23rd. Given that the characters, dialogue, events, and details sound mostly modern and take place within just a few weeks, we can assume that the year is intended to be at or around the time the book was published. It has neither a futuristic tone, nor a does it give evidence of being set too far in the past.
Certainly the 1950s and 60s are frequently portrayed in books and film with carnival scenes as a primary social gathering for kids and teenagers. Something about the romantic but dark and mysterious air that surrounds a traveling carnival makes a great setting for any story that centers around the lives of teenagers. Likely the author meant to appeal to his audience by giving them a story that sounded like it could happen to them.