A personal note, for a couple of years I used to work about 40 hours in 4 days, with 5 hours commute on the first and last day. Those four days I had no real life whatsoever, followed by three days of weekend. I could do it but it wasn't nice.
A 36-hour workweek is entirely feasible
Quite a lot of industrialized countries are worried about the long-term effect as computers displace the workforce from the service sector, just as they have done from industry. Having part of the population in long-term unemployment or McJobs isn't good for society, better use the productivity increase from modern technology to cut everybody a little slack.
A 9-hour workday is also feasible
Plenty of people work that much, for sustained periods. Depending on what they do, the productivity of the 9th hour might not equal the productivity of the 5th or 6th hour ...
Who gets to work weekends?
So now you have everybody at home from Friday to Sunday, right? Well, they'll expect that bakers are open on Friday and Saturday. And grocers. And barbers. Government offices, too, since employed people can't file their tax forms during the working week.
Pretty soon you have so many exceptions that the "rule" is more of a guideline. It might apply in the industrial sector, but not in the service sector and not in agriculture.
What happens to web-shops? Are they allowed to be open? Would that mean the big internet business has another edge over local shops? Especially if parcel delivery is allowed to work and the parcels can be prepared abroad.
It will take some time until foreign business partners get used to the fact that nobody answers the phone for three days in a row. They might take their business elsewhere.