We keep as many courses as we can open for registration so that students can audit the material after the course is finished. Unfortunately, we can't do this for all courses and any which show "Not currently available" instead of an enroll button are closed and cannot be accessed. The course may be opened again in the future.
For updates on new and repeat courses keep watching for our newsletter updates via email and checking our course catalog periodically. You can also keep up-to-date by "liking" edX online on Facebook or following us on Twitter.