What is ironic about Scout's observation that the mood in the town was that of a gala?
To Kill A Mockingbird
It is best to start with the quote to get a sense of the context. Here is the quote:
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.
The irony of the description is obvious from the outset of the words. The masses of the people were there not for a party or a celebration, as the description would suggest. The people have gathered for the trial of Tom Robinson. This shows that the people were there to see, to be seen, to have a good time, and perhaps even to be entertained. There is no indication that the people were there to see a trial or to seek justice. All of this sadly shows that the mindset of Maycomb is deranged.
Atticus actually makes this very point, as he says that Maycomb has a disease when it comes to racial issues.
The passage, therefore, is saturated with irony. The juxtaposition of the gala culture of the text is completely incompatible with the unjust accusation of an innocent black man.