Every Medicare beneficiary has three types of enrollment periods: IEP, AEP, and SEP. IEP is the Initial Enrollment Period. You will have an IEP that begins three months before you turn 65 and ends three months after. You must enroll during that time. Then, every fall, you will be able to make changes during AEP, the Annual Enrollment Period, which runs from October 15 through December 7.
Some (but not all) beneficiaries will have an SEP, or Special Enrollment Period. Events like moving to a new service area or gaining or losing an income source can give you an SEP. Anyone who has Extra Help or a Medicare Savings Program has a continuous enrollment period. Those with a continuous SEP can enroll or make changes once per quarter during the first three quarters of the year.