Enrollment periods are certain periods of time when you can join the Original Medicare program, or elect a Medicare Advantage Plan, Medicare private drug plan (Part D), or supplemental insurance plan (Medigap). See also: Fall Open Enrollment Period, Medicare Advantage Disenrollment Period, General Enrollment Period, Initial Coverage Election Period, Initial Enrollment Period, Initial Open Enrollment Period, Open Enrollment Period, Special Election Period, and Special Enrollment Period.