Class always officially start on a Monday. See the academic calendar in the catalog for official start dates. You will have full access to the course the Friday afternoon (Mountain time) before the start, which is called the open house period. You may access the course, download the syllabus, and start posting as early as Friday afternoon before the official start date for the term.
However, in order to confirm your attendance, you will still need to post something in your course on or after the official start date. Posting during the open house period does not count toward your first week's attendance. Posting in the official week 1 timeframe officially indicates your intent to continue in the course.