This occurs during chapter 16. The whole town shows up for the trial as if it is the most important social event of the decade. Scout describes it as this:
It was a gala occasion. There was no room at the public hitching rail for another animal, mules and wagons were parked under every available tree. The courthouse square was covered with picnic parties sitting on newspapers, washing down biscuit and syrup with warm milk from fruit jars. Some people were gnawing on cold chicken and cold fried pork chops. The more affluent chased their food with drugstore Coca-Cola in bulb-shaped soda glasses. Greasy-faced children popped-the-whip through the crowd, and babies lunched at their mothers’ breasts.
This quote proves that folks had come prepared for a long day. The only reason to pack in food and bring the little children must be the fact that everyone was there and planned to be there for a long time. The people are sitting in anticipation and awaiting the opportunity to get good seats in the courthouse. It is as if they were waiting for a popular concert to begin.
I think it is also important to notice that the white folks and black folks did separate themselves and Scout as narrator points this out as she spends time describing the black folks in a separate paragraph:
In a far corner of the square, the Negroes sat quietly in the sun, dining on sardines, crackers, and the more vivid flavors of Nehi Cola. Mr. Dolphus Raymond sat with them.