I can say from my perspecitive about formal night is it is what you want it to be. You can wear your best cocktail dress, or do what I did, my dressiest clothes for work--a pair of pants and two blouses. Many wore a nice dress (like to a fine restaurant), while others wore casual clothes. One couple we met didn't bring anything vaguely dressy at all. They brought clothes that they would wear to a decent restaurant and left it at that.
I say, make it what you want, they won't turn you away and if you are happy, everything is groovy.