The General Enrollment Period (GEP) is the time period every year from January 1 to March 31 when you can enroll in Medicare Part B for the first time if you missed your Initial Enrollment Period (IEP) and do not qualify for the Part B Special Enrollment Period (SEP). When you enroll during the GEP, coverage begins the first of the month after you enroll, and you may have to pay a Part B late enrollment penalty.