i went to a french/swiss wedding, and girls were wearing pretty much the thing you'd see at any wedding. Hats feathers, nope didnt see any of those, but the girls all looked fantastic pretty dressed up, but it was a bit of a mix between casual and formal....
Wont think you'll be needing a hat, more like a bonnet in the -5 it will probably be....

enjoy! Oh and if its anything like the one I went to, I would suggest not eating anything for about 3 days before it so you can stuff yourself crazy at it....there was alot alot of food....and alcohol. Started at 4pm and finished at 4am, prepare for a loooong day/night of partying.