The negative effects of fatigue are specific to the activities that created that fatigue in the first place. This means that ideally, neighboring workouts should be as dissimilar as possible, given whatever constraints you're working under. In this way, the fatigue from Monday's session will have the most minimal possible negative affect on Wednesday's workout.
Example: Leg Specialization
Imagine that you're determined to improve your leg size, and you've determined that you should train legs three times a week. The "Contrast Promotes Recovery" principle dictates that these workouts should be as dissimilar as possible in order to promote maximum recovery from session to session.
There are a few different ways to accomplish this, foremost among them being varied exercise selection and loading parameters. Here's how you might put this principle into action on a practical level:
In this example, both exercises and loads change significantly from workout to workout. Another approach might be to alternate between knee/quad-focused exercises in one session with hip/posterior chain-focused exercises in the next session.