A bride in September, always a bride?

A highschool buddy of Josh is getting married in September. We are going. The invitation said to BYOB and the website said it was a Medieval wedding. I think it was because the proposal happened at Excalibur!

We will give a gift of money or a gift certificate to Walmart. I am just not sure if I should break out the polyester Lady Marian Dress or dress Montana casual.