In order to give fair notice to instructors, if a class is going to be cancelled due to lack of enrollment, cancellation is done a couple days before the class is scheduled to start.  However, sometimes enough people sign up at the last minute that the class could have run. Unfortunately, once the decision has been made to cancel a class, again, in fairness to the instructor, that's it. So, if you don't want to see a class of your choice cancelled, remember to register early!