In the days leading up to the event, Dunn and his team worked tirelessly. They created more evidence and carefully coached the 'witnesses' who would testify against Percival.
As for Johann, he too was busy with the preparation of the banquet. It was held in honour of Adam's death. At least, that's what Johann propagates to the people.
Nevertheless, as he expected, many shamelessly agreed to attend the banquet.
Despite them never treating his dead grandson nicely, they readily accept the invitation.
For most people, the reason for the banquet is not important. The point is that it's a banquet, so why should they let go of an opportunity to socialize?
The lure of delicious food, drink, and social interaction was irresistible.
Plus, these people don't really have anything to do.