Since most schools should be back in from Winter Break, I believe a lot of college schools are still out (I know mine is!) I think your busiest time will be on the weekend, and Fridays are a bit busy but not bad until the evening. I prefer going on a Wednesday or a Tuesday or Thursday if possible. But I think you'll be fine with whatever day you choose since you are going at a time where I think it would be less busy
Unless MK Day is during that trip, I honestly can't remember when it is...