Self-paced courses do not follow a set schedule and are open for 3 to 12 months. Course materials are all available as soon as the course begins. Assignments do not have start or due dates other than the official end date of the course.
Self-paced courses have a "Self-Paced" indicator in the edX course catalog, along with a start date if the course has not yet begun.
In most self-paced courses, you will receive a certificate as soon as you have a passing grade in the course, even if you have not completed all course materials.