When can I start editing next semester's course sites?

A course in Canvas is available to YOU (as a faculty member) to begin working on roughly 120 days before the start of the term in which it's being taught. To see a future class, click on the Courses link on the far left navigation bar in Canvas, and from the ensuing menu click All Courses. On the resulting screen, you will see a listing for Future Enrollments. To set one of those classes so that it appears on your Canvas Dashboard when you next login, simply click the star icon to the left of the course name, turning it a yellow color. The next time you login, the course should appear on your Dashboard.