Here's an odd question: my wife works 80% (she's off on Tuesdays). Her contract states only "80%", without any reference to how that percentage is calculated. The general working conditions state that the details are handled individually (hinting at the fact that you can arrange with your boss which day of the week you don't work, I guess).
All employees at my wife's workplace (a Kantonspital) punch in, using an electronic badge, so when they swipe the card, they can see on the display how many hours they have "accumulated" during the current month. It displays any overtime you have worked (they are not paid for it, so they can compensate), or how many hours they owe, in case they came in late/left early some days.
Last week we had a long weekend (Friday was a holiday), so she only worked 3 days during that week. And here's the catch: the hospital system states that she now owes hours, because she didn't work on Tuesday or Friday. So according to this "calculation method", she needs to switch her day off from Tuesday to whatever weekday a holiday happens to fall on that week.
Does that sound normal in your region ? I imagine there are different regulations in each Canton and each employeer, but this case is just plain weird. Imagine you work 60%, and on a certain week there are 2 holidays. How can you possibly compensate, short of working 16 hours for 3 days ?