Would it hurt my child as far as the psychological (sic) taking the medication only Monday - Friday and not on Saturdays, Sundays, Holidays & Vacations? I am worried that this unbalance in the medication is not good for the chemical balance in his brain.

It's perfectly okay to use Ritalin during the week or not on weekends. The problems of ADD if seen more as a "living imbalance" and not a "chemical imbalance" are those of a child not meeting the demands and expectations of his environment (school or home). If the demands are less say on weekends or vacations there is no need to enhance his performance by using medication. There is no danger in taking these breaks. At the same time there is no evidence that taking the medication 365 days a years is worse than intermittent use. My only concern about intermittent use is when someone only occasionally uses Ritalin, for example, to cram for an exam or stay up all night. I believe that only exacerbates the disorganization and procrastination that goes with adult ADD and should be discouraged.