Background info on what they're celebrating at the banquet:
Wayrest was taken over by a coalition of pirates, known as the Corsairs, in 4E 188. It has been run as a republic after that, though it was treated as a pariah state by the rest of High Rock. However, the other High Rock monarchies are divided and do not have central authority (they're loyal only to the Emperor in Imperial City), thus they can't find a common solution to deal with Wayrest.
However, at the beginning of 4E 205, the Emperor called all leaders of High Rock to a meeting. They're to elect a high king to represent the entire province, so that it doesn't fall apart like Skyrim, or become individually susceptible to manipulation by the Dominion. King Ferrand Bellemont of Daggerfall is chosen for the high king.
One of the first thing Bellemont ordered is for the Corsairs to leave Wayrest (likely due to the Corsairs raiding Daggerfall ships). When the Corsairs refused, Bellemont managed to convince other High Rock leaders to form a military coalition, so that they can liberate Wayrest by force. They began a naval blockade and a land-based siege against Wayrest on Midyear. The coalition finally captured Wayrest at the end of Sun's Height.
However, the coalition couldn't agree on the future leader of Wayrest. Bellemont wants to install his son, Prince Gregory, as the ruler. But towards the end of the siege, a previously obscure bastard child of the pre-Corsair monarchy, Everard III, joined the coalition, leading a mercenary company. Everard III is being championed by Prince Narcisse Septim-Vincens of Camlorn (a rival of Bellemont) as the alternative king of Wayrest. Northpoint backs Prince Gregory, while Jehanna supports Everard.
Currently, on the 16th of Sun's Height, both sides have thousands of troops occupying Wayrest. Evermore, whom had only sent medical support to the coalition (because their military is much smaller), is neutral on the dispute. The Duchess of Evermore, Emmeria Guimard decides to host a banquet, in the name of celebrating the liberation of Wayrest. Various prominent figures are invited, but beneath the surface, Duchess Guimard is bringing the two sides to the negotiating table, along with third-party mediators, in hopes of creating a compromise (and increasing Evermore's influence in the process).