Hi, Sue! The best days to visit which parks can be a tricky science. The crowd level really depends on a lot of different factors, including the time of year, special events, extra magic hours, even the weather!
I love the site www.touringplans.com to help me plan which days might be the best for each park. While it may be difficult to account for the impact of things like free dining or the buy four, play seven promotion, I have always found them pretty accurate in their recommendations for which parks are better on which days.
If you have park hoppers, I like to hit the extra magic hours in the morning, then hop to a different park later in the morning. Magic Kingdom early on a Saturday is also one of my favorite times. I think many people use Saturday as a travel day, so I like to go when they open in the morning. I feel like I get quite a bit done on Saturday mornings.
January is typically a month where you'll see lower crowds anyway, but generally, you'll find bigger crowds in the parks with extra magic hours. Beyond that, I rely on Touring Plans.
Enjoy a fabulous vacation!