It's President's day weekend and lots of northern schools will be on break, so I'm not sure it's possible to say what will be the "least crowded day" at any park. Your best bet may be to pick up the park maps as soon as you arrive at WDW and then spend a few minutes reviewing them and deciding which shows and attractions look good to you. Then, based on what you decide, devote a day at each park to hitting those attactions, but try not to freak out if you cannot get everything done. WDW is huge and it is hard to do everything, even if you had two weeks to do it. Most of all, just try to have fun and not get stressed over lines and waits for buses, etc.