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 11:59pm on Monday, February 1. You should register as early as possible after the start of your appointment, but the continuous availability of the system until February 1 should help you if you need to make changes in your schedule.