The most troublesome thing in general is the complexity of simple questions. Personally, you'd better ignore the day of the week. Eight people each line up for one day, from the first day to the seventh day. The eighth day is next Monday, and then the first day will continue. Generally, when cleaning, the group or the other half will be cleaned next time. So you need to unify the duty list for one semester, and then look at the people on duty against the list.
I hope it helps you.