That is just how it is. If you're confused, it's because you're looking for a system and not finding one. But prepositions are like that. There's not really any satisfactory explanation, because nobody sat down and devised the language. It just happens that Swedes find many good uses for på. The only vaguely helpful thing I can say is to just think of it as the English word on, and accept that the Swedes prefer to say "on the school" rather than "at school".
In this case Duo accepts/suggests the translation "during workdays" which frankly comes across as a slightly unnatural to me. I'd say that "on workdays" sounds much more natural in English, and then it's actually the same preposition as we'd use in English (på = on). I'm sure that's what I wrote and was also accepted.
Prepositions have a habit of confusing. I would not use in. I would use on with days of the week.
As a non native english speaker I'm having sooo much trouble understanding the particles. I mean, I know the theory and why and how. It's just not natural for me to use them at all. Lately I got it in Swedish, but now I have more troubles in English. And in tasks like this I forget to write them. Does anyone have any idea how to get on the bottom of this for those whose native language has cases(forms) and to be more specific, slavic origins. It's making me crazy.
This may just be me, but as a native English speaker, 'during' in the English translation for this feels odd and out of place. At least in standard American English, 'on' is much more common here (and also arguably closer to how 'på' gets used in a number of other contexts in Swedish).