I don't know about everyone else, but my evenings are far from entirely "personal time." Chores take up most of my evenings, so really I get maybe 1-2hr of personal time on work nights. People with kids who do sports probably get next to zero personal time, beyond maybe vegging out on YT/FB for an hour before bed.
Weekends are the only days where I generally "plan" to do things, and even then, Sundays are half reserved for chores like mowing/shopping. Having an additional weekend day that I can plan for hobbies would be amazing. For me, it's honestly akin to doubling my free time.
I think you might be surprised to see how all that "extra" personal time can get absorbed by chores as well, ala Parkinson's law[1]. Chores are personal time unfortunately.
If you sleep 8 hours a day and work for 8 hours Mon-Fri, that's:
- work: 40hrs
- personal: 72hrs
- sleep: 56hrs
Switching to a four day work week would thus result in a 20% reduction in work hours but only a 10% increase in personal time.
I'm going to assume you weren't including non-work hours if they were on "work days", but that seems silly if you have a 9-5 job.