Why were all the people going past the Finches' house in To Kill a Mockingbird?

You don't specify in which chapter this takes place, but
I'm going to assume that it is Chapter 16--the day of the trial of Tom Robinson.
According to Scout, the whole "county went by us." Aunt Alexandra had already ordered
Jem and Scout to "stay in the yard today," and they did--for a short while. A "steady
stream" of people passed leisurely by the Finch house on their way to the courthouse.
People walked, rode on horseback, and sat in wagons. They particularly enjoyed the
banter between Miss Maudie and a group of stern Christians who commented on her
"vanity." They "held off until noon," when Atticus came home for lunch. Then they
gathered Dill and headed to the courthouse square. It would be a day they would never
forget.