Actually 일주일 means one week, so it has different meaning from 주일. 주일 means week from Monday to Sunday (but I believe not many people say "주일", but "주" instead. So, they are not really interchangeable. For example, when you want to say as following, -The last week of this month, 이번달의 마지막 주(주일) - One week, 일주일 - Two weeks later, 2주일(주) 후 But 주일 is also regarded as Sunday in Christianity. - Going to a church 주일에 교회에 가다. Hope this may help you!