I've actually had good visits to MK on both Saturdays and Sundays. I avoid Mondays. But realize that a variety of factors can affect the crowd levels. Decades ago, for our first visit to MK, we planned for a Friday because Birnbaum's guide (remember that book?) indicated it was slow. All day as we were maneuvering through the park, we kept thinking to ourselves that if the crowds we were seeing were low, we didn't want to see MK during a busy time. It turns out that was the weekend they were saying farewell to Main Street Electrical parade (the first time) and there were actually vehicles parked under the monorail because there was no more room in the parking lot!!!
Have fun!