1. Initial Enrollment Period (IEP)
Your personal seven-month launch window: the three months before your 65th birthday month, your birthday month, and the three months after. Enroll in the three months before your birthday month and coverage starts the first of your birthday month; enroll later in the window and the start date slips.
| When you sign up (relative to 65th birthday month) | When Part B starts |
|---|---|
| 1 to 3 months before | First day of your birthday month |
| During your birthday month | First of the following month |
| 1 to 3 months after | First of the month after you enroll |
2. Medigap Open Enrollment
Six months starting the month you are both 65+ and enrolled in Part B. Carriers must accept you with no health questions. This window does not repeat. If a Supplement is on your shortlist, this is when to buy it; details in the Medigap lesson.
3. Annual Enrollment Period (AEP): October 15 to December 7
The fall window everyone advertises. Anyone on Medicare can join, drop or switch Advantage and Part D plans, effective January 1. Plans re-file their networks, formularies and copays every year, so the plan you love can quietly change underneath you; your Annual Notice of Change letter, mailed in September, is the early warning.
4. Medicare Advantage Open Enrollment (MA-OEP): January 1 to March 31
A do-over window exclusively for people already on an Advantage plan: one switch to another Advantage plan, or back to Original Medicare plus a Part D plan. Medigap underwriting may still apply on the way back, which is why we treat this as a safety net, not a strategy.
5. General Enrollment Period (GEP): January 1 to March 31
The catch-up window for people who missed their IEP without other coverage. Coverage begins the month after you enroll, and the Part B late penalty usually rides along.
6. Special Enrollment Periods (SEPs)
Life events open personal windows: leaving employer coverage (8 months for Part B), moving out of a plan service area, qualifying for Extra Help or Medicaid, plan terminations and more. SEPs are the reason you should never assume you are stuck; ask before you resign yourself to a bad plan year.
The two penalties to respect
| Penalty | How it accrues | How long it lasts |
|---|---|---|
| Part B late penalty | +10% of the Part B premium per full 12 months without coverage after eligibility | For life |
| Part D late penalty | +1% of the national base premium per month without creditable drug coverage | For life |
Write two dates on the kitchen calendar: the first day of the month you turn 64 and 9 months (start planning), and October 15 every year after (re-shop). Every Medicare disaster story we have ever untangled began with a missed date.