You can enroll in the certificate program and start courses at any time in the year (any of the four quarters per academic year) during the enrollment period of that specific quarter.
There is no requirement to "enroll" in the certificate to get started. However, the certificate’s requirements are subject to change, so enrolling earlier in the certificate may allow you to “lock in” your certificate requirements so that you are not subject to possible future changes.
If you are enrolling in individual courses, you can start enrolling for courses once registration for that quarter opens (calendar). Students are allowed to enroll in a course (as long as there are open seats) up until the day before the 2nd meeting of the course (if the course is a standard XL-level course).