As others have said, there are very few predictable times when the crowds will be light in the parks. As theluckyrabbit said, any day when there is steady heavy rain will probably be pretty good. If the rain lets up, then the crowds might show up (I was there one day when it rained pretty steadily in the morning, but let up around 11:00 am and by mid-afternoon, the typical crowds had appeared). So, if you have flexibility to go on very short notice, I'd watch the weather reports for a day when it's supposed to rain steadily all day long.
Otherwise, you'll need to look for a time when there are not holidays, no special event, and no special ticket deals happening. I would nominally guess that mid-week (Tue to Thu) in Jan (outside of MLK week) and Feb (outside of President's Day week) might be your best bet. If you wait to long into March, you'll have to content with Spring Break season (mid-March through mid-April, pretty much whenever F&W is happening). There might be a small window after Spring Break season and before summer starts (late-May), but there are often ticket deals that bring in the crowds.
Once Star Wars Land opens in the summer, I'd guess that there aren't really going to be any low crowd periods for the remainder of the year. There may be random days when the crowds are lower, but no days when there are no crowds.