Depends on age and life circumstances.
M-F is when you go to work then come home and drive the kids around to after school activities (which at high school age often run into the evening hours), then rush home, fix dinner, then try to watch something but you're too tired and it's too late to stay awake. On the nights where they don't have something scheduled, you can go out for dinner or some live entertainment if the kids are old enough to stay home alone.
F-S-S (Fridays straddle both categories because Saturday) is when you catch up on the household chores and then plant yourself on the sofa to work through the DVR and Netflix Disc queue because the last thing you want to do is get in the car and drive some more. And the kids don't have homework due the next day (F-S), so they can take the time to watch the Netflix disc with the family.
Anyone else with a different pattern from the three above (including mine)?