On Friday or on Fridays, which would be the better reply?
Do you have to go to lessons every day?
Not every day, but I have to go on Wednesday and Friday.
Not every day, but I have to go on Wednesdays and Fridays.
If you're describing your regular routine, it's the second one. You go to lessons twice a week, every Wednesday and Friday.
If you're only talking about this week's schedule, it's the first one.