It was lovely - and focused.
Statutory holidays were awkward, though, because then it became a three-day workweek, which was too short. If I were to do it again, I would set it up to work 4 days per week every week - even those weeks with a statutory holiday.
Productivity was very high on the days that I worked.
A three day weekend allowed the first day of the weekend for personal chores and hassles (such as medical appointments and large grocery shops). That meant a two day weekend for fun and recreation, and an extra day for personal chores. The weekend became truly recreational.
Recently I switched from a four day work week to a five day work week. My pile of personal hassles is building up more and more, and I am not sure when I will find the time to complete them. My work days are a bit less focused, because more of my midweek brain is taken up by thinking about personal chores and hassles.
That is, I think, the main point: a four day work week increases focus. Those four work days (M-Th) became more focused on work, because I was not also thinking about my personal chores and hassles. In the four day work week, if a personal chore/hassle came up, I simply wrote it down to do on Friday (my extra day off). The benefit to focus on M-Th work was substantial.