Wouldn't Monday be a rehab day? That would make more sense, since Sunday is still officially a weekend day, and still a party day. Who would possibly rehab on a Sunday?
"Hey Fred, wanna go to the river today and do a little boating"?
"I can't Barney, its Sunday and I'm rehabbing" Anyone who has a job and has to work on Monday. Unless your job consists of laying by the pool and drinking!