depending on whether the race is important, a major goal, or just for training will depend on what you do. you should plan your training around your races, so that you arrive at your races in good condition.
for anything that isn't a training race, what most people find works very well, is to have the day off 2 days before the race (e.g., day off friday if racing sunday) and the day prior should be an easy spin at recovery levels for 45 - 90 mins (depending on fitness level) with a few race pace type efforts of ~ 30-secs duration (e.g., 3 x 30-secs).
Once you get in (and during the training ride too) you should take in plenty of carbohydrates to replenish your bodys carbo stores (muscle and liver glycogen) as racing is virtually dependent on these stores.
Ric