In general - midweek and Saturdays are cheaper because many business people fly on Mondays and Fridays. However, you won't know for sure unless you price it both days.
We are flying out of MSP in February. I checked for Saturday first, as that was our original plan. Then I checked Friday, it was $60 cheaper per person to fly on Friday instead. So, we added a night to our hotel and we get to start our vacation a day earlier
Incidentally, I also checked for every other day of the week (I played around with all the possibilities for arrival and departure) and Saturday was the most expensive.
For our August trip, Fridays, Satursdays and Sundays are all higher priced. Mondays-Thursdays slightly lower. Really going to be location and date specific.