How Can Flying Midweek Increase Your flight Upgrade Chances?

Upgrade chances are greater—and less expensive—if the carrier is in the habit of flying slow. Tuesday or Wednesday, i.e., off-peak or mid-week flying maximizes chances of getting upgraded to lower fare. These are the best times to get upgraded as there are enough opportunities for airlines to fill low-rate empty space.
If you're going to have to change your flight dates more than any other, remember that. When you change flight date, it will make a dramatic difference in what you can gain and how much you'll be charging.