this is a tough question but there are some basic answers to it.
avoid weekends, they are crowded. people that come in on a friday to the onsite hotels, makes friday afternoons crowded at the parks. sat and sun are a given as any other theme park has more in attendance then also.
if you have a 3 day weekend, avoid the monday
columus day is one i stay at hrh and can tell you on mondays by noon, both parks have their crowds.
i have usually found that IOA has more of the crowds around 11ish
studios around noon or later. many people will start at IOA to avoid long lines for rides and crowds then go over to the studio side after lunch.
i have always had great luck with tuesdays, wednesdays and thursdays at both parks.
i stay on site so don't have the long lines for the rides compared to those that stay off site. even though i use the fotl or single rider lines, i start at the parks at the turnstile at 8:30 am.
you have better chance of less crowds for the days in the middle of the week
larger crowds at the studios in the afternoon i am starting to see more of.
it also depends on the time of the year you visit UO on how crowded the parks can be. when the parks close early, the crowds are low in the parks.