We have not been in September, but we do always hit MK on Sunday morning and find it to be a very nice time to be at the park. Saturday is the day to avoid for the MK. MK is however EE on Sunday now, so i would expect the crowds to grow here as the day wears on. If you are staying on site, I would for sure hit MK at 8:00 on Sunday. If not, you might want to choose another park.
Jordan's mom