You do not need to take all of the courses in a program at once. You can take the courses in a MicroMasters, MicroBachelors, Professional Certificate, or XSeries program one at a time as they are offered.
All courses which are part of a program credential are expected to repeat each year. So, although sometimes all of a program's courses run simultaneously, you don't have to enroll in them all at the same time. You have the option to enroll in just one course and wait for the next sessions before you enroll in the next course.
Most courses only show the currently available session, but some courses show the available sessions over the next year or so, so that you can choose the session dates that work best for you.
If you've purchased a program bundle, please see more information on selecting sessions here: How do I choose a course session?