Ideally, the sooner, the better. Popular classes can fill up early. You can increase your chance of getting in by registering early. Although courses remain on eCourses until the last day you can register, there may be barriers that prevent you from registering at the last moment (see the part about waiting up to 6 weeks for a letter of permission). With this (and other realities) in mind: If you plan to take a course as a visiting student, you should get started four to eight weeks before registration opens at the host institution(s). Because of deadlines and processing time, it may not be possible to register if the course begins shortly.