Grand Canyon is in the early morning or late afternoon on sunny days. The low angled sun rays intensify the hues of the rock layers. The PM is best if your flying out of Vegas since you'll get to see the sunset on the way back and also the Las Vegas Strip lights when landing. Day of week doesn't matter its about when it works for you.
Yes, I don't know what day of week is best (I imagine they're all the same) but you have to do it early morning or at dusk, whichever's easier for you. Midday could be fun if you had no other choice, but it wouldn't compare to the other two options.
I certainly wouldn't do a trip at midday as you'll be right out in the open with hardly any cover from the sun so as others have suggested would opt for morn or dusk.