DS 25 always visits the MK on his last day, even if it is a crowd level 7 on most days of the trip and a crowd level 10 on the last day he does this (so likes to end the trip with the classic park). And we cater to DS when he is traveling with us to some extent. I specifically booked a Friday to Friday trip for the next one we are doing, so DS would have lower crowds on his last day for our next trip (Our last day will be a Thursday instead of the busier Friday, Saturday, and Sunday days). Weekends seem to be busier these days for some reason, from what I have read. In the past I don't think it made as much of a difference as so many were traveling on weekends (more locals??, more mini weekend trips??, who knows).
Oh and on this next trip (six full days next March): DS will be doing four theme park days (one day SeaWorld (Seven Seas Festival), one day Universal park to park, one day Epcot for Flower and Garden, and the last day at the MK). We've been to AK and HS recently, so have seen all the cool new stuff there already FYI. Just letting you know we only do the MK once and do it on the last day.