Initial Enrollment Period (IEP) — the 7-month around 65
Your IEP starts 3 months before the month you turn 65, includes your birthday month, and continues 3 months after. Total: 7 months.
If your birthday is June 15: IEP is March 1 – September 30.
What you can do during IEP: enroll in Parts A and B. Sign up for Medicare Advantage (Part C). Sign up for standalone Part D. Start your 6-month Medigap Open Enrollment window (which runs from when Part B starts).
If you enroll BEFORE your birthday month, coverage starts the month you turn 65. If you enroll DURING or AFTER, coverage may be delayed.
Auto-enrollment — when it happens automatically
If you're already collecting Social Security retirement benefits (or Railroad Retirement) at 65, you're automatically enrolled in Parts A and B. Your Medicare card arrives in the mail about 3 months before your birthday.
If you're NOT collecting Social Security at 65, you have to actively enroll through SSA — online at ssa.gov, by phone at 1-800-772-1213, or at a local Social Security office.
General Enrollment Period (GEP)
If you missed your IEP and didn't have other creditable coverage, you have to wait for the General Enrollment Period: January 1 – March 31, with coverage starting July 1.
Plus, you may owe the Part B late-enrollment penalty: 10% of the standard premium for every 12 months you went without coverage. Example: if you delayed Part B for 5 years without creditable coverage, your premium is 50% higher — for life.
Annual Enrollment Period (AEP) — every fall
AEP is October 15 – December 7 every year. New plan starts January 1.
This is when you can: switch between Original and Medicare Advantage. Change MA plans. Change Part D plans. Drop or add Part D.
AEP does NOT generally let you switch Medigap plans — Medigap is governed by separate windows (initial 6-month Medigap OEP, plus state-specific rules).
Use AEP to re-shop your Part D plan annually. Use it to evaluate whether your MA plan is still the right one.
Medicare Advantage Open Enrollment Period (MA-OEP)
January 1 – March 31. Only available if you're already in an MA plan as of January 1.
What you can do: switch to a different MA plan, or drop MA entirely and return to Original Medicare (with the option to add a standalone Part D).
What you can NOT do: switch between Original Medicare plans, change Part D unless you're moving to/from MA, get a Medigap with guaranteed issue (you'd need state-specific rules to enable that).
Use MA-OEP if you enrolled in an MA plan during AEP and quickly realized it's not right (network issues, prior auth nightmares).
Special Enrollment Periods (SEPs)
SEPs are triggered by specific life events. The most common:
Working past 65 SEP: when you (or your spouse) leave employer coverage, you have 8 months to enroll in Part B without penalty.
Moving SEP: when you move out of your plan's service area, you get 2 months to switch plans.
Plan termination SEP: if your plan is terminated by Medicare or the carrier, you get 3 months to switch.
Eligibility-loss SEPs: lose Medicaid, lose employer coverage, gain extra-help, etc. Each has its own timeline.
5-star SEP: you can switch to a 5-star MA or Part D plan once per year (Dec 8 – Nov 30) outside AEP. Most plans aren't 5-star, but if a 5-star plan is available, you can move at almost any time.
The Medigap Open Enrollment Period
Separate from everything above. Your Medigap OEP starts the month your Part B starts and runs 6 months. During this window, you have GUARANTEED-ISSUE rights — any Medigap carrier must sell you any plan at the standard rate.
After this window, in most states, carriers can use medical underwriting. So this 6-month window is genuinely your one shot to lock in cheap Medigap regardless of health.
If you turn 65 already on disability Medicare, you get a SECOND Medigap OEP at 65. This is important — it can give you guaranteed-issue access you didn't have under disability rules.