I usually do some small exercises just to check the last things and sensations before the competition ;)
If you really want to ride your horse, do simple exercises that won't require a lot of effort and don't ride more than 30 minutes :)
To me, it is also a good idea to let the horse rest and be ready for the D-day.
I agree with Ryan. I didn't ride the day or two before a show because I wanted my horse to be well-rested and ready.
The day before I almost never work my horse! I let him in his field with his buddies. It is the best way to keep him concentrated and in good shape the D-day!
I always do exercises the horse perfectly does so he is confident for the competition. I do not jump the day before a competition, I prefer to work easy dressage exercises and let the horse in the paddock, so he can enjoy the grass and relax.