We started doing that literally as the first Covid lockdown hit. We always worked 36 hour weeks, but we just compressed them into four 9-hour days instead of five 7.2-hour days (which isn't quite what these trials were doing, but it's close enough that we're not complaining).
Honestly, it's amazing. It really doesn't feel much longer to be working, and having a three day weekend seems to be making everyone happy.
Bonus for me is that there's a handful of people that want to stick with a five hour day, and so as part of a support team, one of us had to be available on Fridays for them, so I actually get Wednesday off instead. So a two day week, a one day 'weekend', a two day week, then Saturday and Sunday.
It's been great for mental health as well as scheduling holidays or just getting stuff done. Work life balance has never been better.
The only downside is that job hunting is now really hard, as anyone that wants me in an office five days a week can jog on.