Missing a Medicare enrollment window can cost you thousands in penalties — permanently. This guide covers every enrollment period, when each one opens, and exactly what you can do during each.
| Period | When | What You Can Do |
|---|---|---|
| IEP (Initial) | 7 months around turning 65 | Sign up for Part A, Part B, Part D, MA, or Medigap |
| AEP (Annual) | Oct 15 – Dec 7 each year | Switch MA plans, add/drop Part D, switch to/from Original Medicare |
| OEP (MA Open) | Jan 1 – Mar 31 each year | Switch MA plans or drop MA to return to Original Medicare + Part D |
| SEP (Special) | Varies by qualifying event | Make changes outside normal windows (moving, losing coverage, etc.) |
| GEP (General) | Jan 1 – Mar 31 each year | Sign up for Part B if you missed your IEP (penalty applies) |
| Medigap OEP | 6 months from Part B start | Buy any Medigap plan — guaranteed issue, no health questions |
The names are confusing by design — or at least it feels that way. Here's the simple version: IEP = your first chance. AEP = your annual chance. OEP = your January do-over. SEP = your life-change chance. GEP = your last resort (with penalty). The Medigap OEP is the one nobody tells you about — and it's the most important window you can't get back.
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Your IEP is the 7-month window centered on your 65th birthday: 3 months before, your birthday month, and 3 months after. This is when you should enroll in Part A, Part B, choose a Part D plan or Medicare Advantage plan, and consider Medigap.
Why it matters: If you miss this window and don't have creditable coverage, you'll face permanent late enrollment penalties for Part B (10%/year) and Part D (1% of base premium/month of delay).
If you enroll in the last months of your IEP, your coverage may not start until months later — leaving a gap. Enroll in the first 1-2 months of your IEP for coverage starting the month you turn 65.
October 15 through December 7 each year. Changes take effect January 1.
During AEP you can: switch from one MA plan to another, switch from Original Medicare to MA (or vice versa), join, switch, or drop a Part D plan, or switch from an MA plan that includes drugs to one that doesn't (and add a standalone Part D).
You cannot buy or switch Medigap plans during AEP (those have separate rules).
AEP is when the TV ads go crazy with Medicare Advantage marketing. Don't get swept up. Instead, use this time to compare your current plan against alternatives using Medicare.gov/plan-compare. Enter your drugs, check your doctors, and see if a different plan saves you money. Fifteen minutes of comparison shopping can save hundreds.
January 1 through March 31 each year. This is your January "do-over."
If you're in a Medicare Advantage plan and realize it's not working — wrong network, drugs not covered, unexpected costs — you can switch to a different MA plan or drop MA entirely and go back to Original Medicare + Part D. Changes take effect the first of the month after you enroll.
Note: If you're on Original Medicare, the OEP does NOT let you switch to an MA plan. That's AEP only.
SEPs let you make changes outside the normal windows when you have a qualifying life event:
Moving to a new area (your current plan may not be available)
Losing employer/union coverage (8-month window for Part B; 63-day window for Part D/MA)
Qualifying for Medicaid or Extra Help (quarterly SEP: can switch plans Jan–Mar each year)
Dual eligible (quarterly SEP throughout the year)
Getting a qualifying chronic condition (one-time SEP to join a C-SNP)
Plan contract violation (your plan isn't meeting its obligations)
Leaving incarceration, gaining lawful presence, or other federal events
January 1 through March 31 each year. Coverage starts July 1.
This is your last resort if you missed your IEP and don't qualify for a SEP. You can sign up for Part B during GEP — but you'll pay the late enrollment penalty (10% per year you could have been enrolled but weren't), and your coverage doesn't start until July 1, leaving a gap.
Unlike a one-time fee, the Part B late enrollment penalty is added to your premium every month for as long as you have Medicare. A 2-year delay means 20% more — forever. This is why enrolling during your IEP is so critical.
This is the enrollment period nobody talks about — and it may be the most important one.
Your Medigap Open Enrollment Period is a 6-month window that starts the month your Medicare Part B is effective. During this window, you have guaranteed issue rights: insurance companies must sell you any Medigap plan they offer, cannot charge you more based on your health, and cannot deny you coverage.
After this window closes, insurance companies can medically underwrite you. If you have health conditions, they can deny you or charge significantly higher premiums. Some states have additional protections, but most don't.
This is the one I wish every single person understood before they turned 65. The Medigap OEP is a one-time guaranteed window. If you skip it because you chose Medicare Advantage, and later decide you want Medigap, you may not be able to get it — or it could cost you double or triple. I've seen people locked out of Medigap because they didn't understand this window. Think carefully before letting it close.
Medicare enrollment timing is genuinely confusing — even for experts. Chapter's licensed advisors can review your specific situation, tell you exactly which window you're in, and help you make the right choice before it closes. Free, no obligation.
Chapter's advisors are licensed, independent, and don't charge you anything — they're paid by insurance companies, not by you.