It's a big celebration to mark a milestone for a club I belong to. We've got a beautiful venue and would like a dress code that means "made a bit of effort". Black tie would be too much and put off people who aren't accustomed to those kinds of dos though. It's in he evening in summer if that makes a difference. Fancy buffet and dancing.
What I'm envisaging is men in smart jeans or nice trousers with a their going out shirt and women in cocktail dresses or similar (I think women end of go more dressy anyway?)
Anyway, what's that called?