David,
Some will be wearing button down shirts and ties. Others will be wearing button down shirts without ties. Some will be wearing their favorite Monon T shirt.
Some gals will be wearing dresses and some will be wearing their favorite Monon T shirt. Jeans are acceptable also. Basically it is up to you. It depends on how special you personally want the night to be.
I will be wearing a tie myself and the gals at my table will be wearing dresses.
Pete