If I remember correctly (I hope I am it seems like I cruised so long ago and it was only Dec. on the Valor)... the Past Guest Cocktail Party was not on a formal night. It was just a regular dinner night and I wore my black dress pants with open toe shoes and a cute baby doll type top.
I do remember that they served way better drinks (and I do not really drink that was actually the first time and I'm over age by a few years ).
Also it was way better than the Captain's dinner cocktail parties, and you actually get to see the Captain and he is/was very entertaining.
Hope this helped a little...and please forgive me if I'm wrong!
The past guest party is not on formal night. If I remember correctly, it is on Thursday night on the five day cruise with Carnival. Meet the captain on formal night is a totally separate party. I never dress formally for the past guest party. I usually wear a sundress or capri outfit to it. Some dress up more than others, but it isn't mandatory.