I agree with Link10103, as far as massage, spa, etc. That will definitley help to relax your body.
As far as relaxing the mind: prayer. You can place anything and everything in prayer (hopefully your prayers are righteous lol). Asking God to help you relax is not something He looks down upon :) Ask Him to bless you with a peace that surpasses all understanding and comfort from the Holy Spirit. See it while you pray and believe He'll answer you.
No matter what you decide to do the day before whether it's spending your day in the company of good friends &/or family, alone in a relaxing environment (quietly at home reading, writing, watching movies, etc) or in nature (at a park, running or cycling, etc.) I would steer clear of overly using alcohol or doing anything that will have you up late - you dont want to be tired or groggy in the morning, not fun lol
I would try to find things now that are relaxing to you, make a list, and the day before the wedding spend the day completing them. Keeping yourself busy will help.
Congratulations! God bless you.