Thursdays and Fridays too. The last two weeks they usually include Wednesdays on the list.
Place is jam packed, even on the week days. I'd suggest you plan a week day trip, like Wednesday, or Thursday.
The good news, is that the day time crowd is light during the week (Wed/Thr). The locals come out at night.
Visit IOA during a week day, have your HHN ticket in advance, then stay in the park when they clean it out around 4pm. That way you can do a couple mazes before they let the crowd in.