Your character can get in the banquet through one of three ways.

1. Attending as a guest, under a fake identity
2. Disguised as a servant and go in the back
3. Shipped into the cargo room in box

Keep in mind that choices 1 and 2 means you must wear appropriate attires and can only carry concealable weapons. You can retain your kit with choice 3. 

Reserve your role within two days, or I will assign it to you.