Why 'or'? Ukr. 'і' is 'and'.
In English, in this context "not working on Monday or Tuesday," it means essentially "working neither Monday nor Tuesday." That is the same meaning as "not working Monday and Tuesday." Duolingo, however, should not mark "and" as incorrect.
Дякую! Дуже цiкаво!
У понеділок і у вівторок should be on Monday or on Tuesday
і = and.
чи = or.
They did not work on Monday or Tuesday.
They were at home both days.
They did not work on Monday and Tuesday.
They were at home on at least one day.