A Roman Carnival has been defined as an event which resembles Mardi Gras. A tradition that dates back to the Middle Ages, this celebration is, to this day, a popular event in Italy, and other countries such as Brazil (whose carnival in Rio is the largest in the world).
Traditionally, Roman carnival consisted of a large public celebration that lasted 8 days, ending the night of Fat Tuesday, the day marking the beginning of Lent . . . During Carnival, the most famous streets and squares would be filled with spectators captivated by the shows, music, masked processions and stands selling goods.[http://www.welcometorome.net/en/about-rome/roman%20culture/roman-carnival]
Miss Maudie's allusion is an apt one, as people from all over the county come to Maycomb's square where the courthouse is located. The parade of odd folks in wagons and other residents of the city set up "picnic parties" on the lawn. When Dill asks Miss Maudie if she is not going, this sensible woman answers that she refuses to engage in such a morbid activity as "watching a poor devil on trial for his life." (Ch.16)
Interestingly, too, is the fact that during the Roman carnival, pigs were herded to an area where there was a cliff. There these animals were forced to jump to their deaths, and the Romans were allowed to collect them and take them home to eat. This practice suggests the concept of a sacrificial animal, which is not unlike what Tom Robinson later becomes for the Jim Crow Southern town.