Well... it's more of a dance party at an art gallery than a wedding. Like... yes, we will be getting married, but, no - there won't be any tables. So - I would say, dress nice - but you don't need to wear a suit or a a dress with sparkles on it (unless you want to). Dress like you're out to have fun, but not at your dad's wedding. Don't dress like you're going to a job interview, but maybe dress like you're going to go get a martini - although I don't think we're going to have martinis at the wedding, but that would be nice. Now I want a martini.