You must have a valid enrollment period to enroll in a Medicare Advantage plan. Valid enrollment periods include the following:
Initial Enrollment Period (IEP): Three months surrounding your Medicare effective date
Annual Enrollment Period (AEP): October 15 through December 7
Medicare Advantage Open Enrollment Period (MAOEP): January 1 through March 31
Special Enrollment Period (SEP): For most SEPs, you will have to select a new plan two months after a qualifying event.