Your body NEEDS Rest & Recovery between workouts for a number of reasons. One is to prevent over-training and probable injury. Two is to allow the muscle to grow, otherwise it will break down. As for the other reasons, it's that "trust" thing again - just do it!

As mentioned elsewhere, you will be doing 2-3 strength workouts and 2-3 cardio workouts per week (depending on your level). It is thus a good idea to alternate the cardio workouts with the resistance training workouts.

Listen very carefully to your body. If each workout feels more tiring than the previous one, you probably need another day off between workouts.