I believe the author chose the setting of Halloween night because of its religious roots and generally creepy nature. Halloween is an exciting night for children, dressed in costumes and sometimes unrecognizable to others, but the holiday will forever be associated with evil and chicanery. With Halloween night as a background, Lee uses the darkness to enhance the suspense. Maycomb is supposed to be safe this year for Halloween--the Barber sisters had been the subject of a prank the year before, and it was decided that a Halloween pageant at the school would be a better alternative to trick-or-treating or worse. Although the children profess no fear of their walk to and from the school auditorium, it is filled with uncertainty and surprise. The theme of the supernatural is evident in the talk of ghosts, "Haints, Hot Steams, incantations, secret signs...," and the children hear the warning of "a solitary mocker" crying from the Radley oak. Aunt Alexandra has had a premonition--"somebody just walked over my grave"--only hours before. A feel of foreboding exists: Scout is late on stage, she forgets her shoes, she decides not to remove her cumbersome "pork" costume, and the children turn down an offer for a ride home--important actions that not only lead up to Bob's attack but which could not have occurred without the pageant on Halloween night. 

Harper Lee  in her timeless novel To Kill a Mockingbird provides the perfect setting for the fight and later killing of Bob Ewell, the antagonist of the novel---All Hallows’  Eve or Halloween. The former is celebrated in many countries and is dedicated to honoring the dead.  The United States has approached the holiday “tongue in cheek” with costumes, tricks, and treats.  It is considered a  “fun” holiday. 

The author’s use of this holiday and the pageant gives the children a reason to walk through the scary school yard and have Cecil Jacobs  join them on the way to the program.  This incident does foreshadow the surprise attack by Ewell on the children.  Later, Jem thinks that it is Cecil who is trying to scare them. It is also the darkest night of the year, and the children have trouble seeing anything.

The weather was unusually warm for the last day of October.  We didn't even need jackets.  The wind was growing stronger, and Jem said it might be raining before we got home.  There was no moon...

"You should have brought the flashlight, Jem."

"Didn't know it was this dark.  Didn't look like it'd be this dark earlier in the evening."

Someone leaped at us.

...Cecil Jacobs jumped in glee. 

The History of Macomb pageant really only serves to have Scout wear her costume.  Because Scout is in the program, her ham costume plays a part because of her inability to see out it.  It is also ungainly preventing her from being able to easily take it off. 

In this more naïve time period, Halloween night allowed imaginations to run wild.  Frightening bird calls; scary shadows; a moonless night; the children’s aloneness---all of these aspects of this night prepare the reader for the attack.  Strangely as Jem hears the attacker coming toward them, the night has become completely silent.  He then can hear the sounds of the man’s shoes and pants as he trails behind the children.

Certainly, the choice of the holiday, the pageant, the children’s lack of adult supervision affords the opportunity for Mr. Ewell’s  attack.  Luckily for the children, Boo was ever present keeping a watchful eye on them.