During photosynthesis, light energy is used to produce ATP molecules which are then used to construct sugar molecules from carbon dioxide and hydrogen obtained from water. Chloroplasts have two outer membranes and several stacks of small disc-shaped membranes called thylakoids (also spelled thylacoids) inside.
Scheduling a week off will give your body time to do this and will also help you return to your workout stronger and ready to challenge your body once again. Many endurance athletes end their season with a week off and many weightlifters will take a week off from their program every four to eight weeks.
The optimal number of days to lift weights per week ultimately depends on your goals and your schedule, but planning your training sessions ahead will allow you to get the best possible results for you. The best number to shoot for is three days a week, with two as a minimum and four as a maximum.