So you are a hardy type who does not let a few mm of rain stop you, here are some fashion suggestions-
Shoes- don't wear your best ones, stay away from suede, straps and crazy height. If you will be indoors all day then you may fit a pair of glam shoes in the hand bag and arrive and leave in more practical ones.
Your hat- Don't get it wet. Take an umbrella, a plastic bag or whatever you need to keep it dry, the sight of a piece of millinery frippery drenched is most unattractive and is irreversible. It will never fully recover.
Everything else. Dress for the day, cocktail dresses in daytime look silly, strappy summer dresses and goose bumps on a cool day look equally awkward.
Have fun, be patient & enjoy the day... back a winner!