Approximately two weeks after the last day of class, your course site is archived as read-only for both yourself and your students. It can still be viewed, however, by heading to Courses on the left global navigation bar, and then selecting All Courses, where it will be linked under the "Past Enrollments" section. However, you can always use the course site in the future to import it into a new Canvas course. To do that, follow the instructions in the PDF linked.