I have been at WDW MLK 'weekend' and it is significantly busier, I seem to remember the Friday wasn't so bad, but the Saturday was busy, we left on the Sunday so couldn't comment on that. As the parks are really quiet at this time of year (practically walk on everything!) it was quite noticeable, but not sure how it compares to really busy times of year , such as July as I've never been then. If you plan a cilling out day, then I would make it this weekend. Maybe try a Water Park (if it's warm enough) or somewhere like Seaworld if you plan to do them.