All instructor-led courses have specific start and end dates during which you can interact with the instructor and fellow students, submit assignments and receive feedback. You cannot access any course until the start date. You will choose which session you want to enroll in by selecting the start and end dates prior to enrollment when there is more than one session available. After the end date of your course, there will be a 30-day period of read-only access when you can still view all course materials.
Any self-paced (or OnDemand courses) do not have start and end dates. These courses are available immediately after purchase and you will have access to go through the course at your own pace.