Sunday has always been and will always be my "day of rest." While in a fitness routine, it is highly recommended that you take at least one day per week to completely rest your body. And rest doesn't mean performing non-weight bearing exercises such as swimming or yoga either...it means REST!
In the most recent edition of Runner's World magazine (August 2010), Ruth England, a coach for Rogue Training Systems in Austin Texas encourages her athletes to have at least one pure rest day each week, even going so far as encouraging them to take the elevators rather than the stairs, etc. A rest day can help you to bounce back even stronger when you are in active training mode and can help clear your mind for the workouts ahead of you.
So don't feel guilty about taking a rest day each week (or even more, especially if you are just starting out.) It is important to listen to your body. It will let you know when to go harder and when to ease up. Train hard, rest hard and your body will thank you!