You have been misinformed. Epcot and AK, except for holiday weekends, usually have their lightest days of the week on Sat and Sun. MGM is also usually OK. MK is crowded on Sat but often somewhat lighter on Sun. This is all relative. During very busy seasons, the parks will still be very crowded on weekends -- just somewhat lighter than weekdays.
The reason for this phenomenom: local residents generally avoid the parks during busy seasons and thus when they show up on the weekends in slow seasons, they are hardly noticed. Sat and Sun are the two busiest days for arrivals and departures into Orlando/WDW and most of those who are traveling on one of those days are not visiting the parks.