The day of the trial might have been a festival in Maycomb. People came from all over the area to see it, on foot, by wagon, by horse. Entire families came and picnicked on the courthouse lawn, and when Scout, Jem and Dill tried to get seats, they ended up upstairs in the balcony with the African-Americans of the community, sitting with Reverend Sykes. This in and of itself shows a great deal about how Atticus has raised his children; they don't think twice about sitting with the blacks of the community, but it is probably safe to say that most of the whites sitting below would have done just about anything to avoid sitting where Scout and Jem were.
There are probably a couple of reasons Lee wrote the trial scene as such an event; likely, in the Deep South during the Great Depression, there wasn't anything else to do. Scout mentions at the beginning of the novel when she is introducing her community that "there was nothing to do and no money to do it with". Secondly, there was probably a great deal of curiosity; after all, when would anyone ever get a chance to see Bob Ewell halfway cleaned up again? And the fact that everyone knew Atticus had every intention of giving Tom Robinson the best defense possible was probably a draw as well; the townspeople, although generally disapproving, probably thought it would be quite interesting to see a man as educated, intelligent, and evolved as Atticus taking on someone as filthy and despicable as Bob Ewell.
The trial itself proves to be heartbreaking to Jem and Scout and serves to underscore the racial hatred, distrust and prejudice that had hung like a storm cloud over the South since the end of the Civil War. Atticus knew he was beaten before he began, but he instructed his children that it was irrelevant; real courage meant "knowing you're licked before you begin but. . .you see it through no matter what. You rarely win, but sometimes you do." Atticus expected a guilty verdict for Tom Robinson, which is exactly what the jury decided, but as Miss Maudie told Scout and Jem, only their father had the capabilities to plant enough doubt to keep the jury in deliberations as long as it did. This was a victory of sorts, because typically in "white accuses black" cases, it was a very quick decision with little or no consideration of anything other than the accuser's and accused's skin color. The black community of Maycomb recognized this, and in what must be one of the most moving passages in literature, they rose as a unit in the balcony to honor Atticus as he left the courtroom as Reverend Sykes motioned to Scout, still seated with him, "Miss Jean Louise, stand up. Your father's passin'."