If you go to class, try not to weigh at home. I used to do this, and my weight would be all over the place from one HOUR to the next. One time I'd weigh myself and I'd have lost, then the next I'd have gained 2 lb! I never weigh at home now.
If you're doing this alone, then just try not to weigh at all until your WI day. And always weigh at the same time. Simple things like having a drink or eating lunch will affect your weight, so try to keep to the same routine on WI day.