That depends what you want to say. on Fridays = every Friday on Friday = scheduled for this Friday

Both are correct; She plays the piano on Fridays - means she plays every Friday, a habit She plays the piano on Friday - in general she plays the piano on a Friday, less of a habit Does he work on Saturdays? - means does he work every Saturday, a habit Does he work on Saturday? - in general does he work on a Saurday less of a habit