Log into SPIRE to see when your registration appointment begins. (Your appointment defines the start and end dates/times you can register for classes. Appointment times are different for different students.) In SPIRE, select "Enrollment", and then "Enrollment Appointment".
Registration appointments run until the Add/Drop deadline at 10pm on Monday, September 15th. You should register as early as possible after the start of your appointment, but the continuous availability of the system until September 15th should help you if you need to make changes in your schedule.