By Marc Gilman
(800) 927-9326 |
Medicare enrollment isn’t a single event — it’s a set of specific windows, each with its own rules for what you can change and when. Missing one can mean waiting months for your next opportunity, or paying a penalty you didn’t need to. Here are the four that matter most.
Key Takeaways
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Initial Enrollment Period (IEP): A 7-month window centered on your 65th birthday — your first chance to enroll.
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Annual Enrollment Period (AEP): October 15–December 7 — the main window for choosing or changing a plan for the upcoming year.
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Medicare Advantage Open Enrollment Period (OEP): January 1–March 31 — a second chance to make one significant change if you’re already on Medicare Advantage.
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Special Enrollment Periods (SEPs): Triggered by specific life events — losing coverage, moving, and several others. Each SEP runs as one fixed, continuous window (usually 2 months), not a fragmented or on-and-off period.
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The rules for low-income and dual-eligible SEPs changed in 2025 — if you’ve heard “quarterly,” that’s outdated information, and this SEP works differently than most (a recurring monthly opportunity, not a single fixed window).
What Is the Initial Enrollment Period?
Your IEP is your first opportunity to enroll in Medicare. It’s a 7-month window: it starts 3 months before the month you turn 65, includes your birthday month, and continues 3 months after.
If you’re already receiving Social Security benefits before turning 65, enrollment is automatic — your Medicare card arrives in the mail 3 to 4 months before your birthday month, effective the 1st of the month you turn 65 (or the month before, if your birthday falls on the 1st). If you’re not yet receiving Social Security, you’ll need to contact Social Security yourself, ideally about 3 months before your 65th birthday, to enroll.
What Is the Annual Enrollment Period?
AEP runs every year from October 15 to December 7. This is the window most people think of as “Medicare enrollment season” — it’s when you can enroll in a Medicare Advantage or Part D prescription drug plan, or change your existing one. Changes made during AEP take effect January 1 of the following year. If you don’t make a change, your current plan generally renews automatically, unless you receive written notice otherwise.
What Is the Medicare Advantage Open Enrollment Period?
OEP runs January 1 to March 31 each year, and it’s specifically for people already enrolled in a Medicare Advantage plan. It allows one significant change: switching to a different Medicare Advantage plan, or dropping Medicare Advantage entirely to return to Original Medicare with a standalone Part D plan. Unlike AEP, changes here take effect the month after you submit them — a change made in January is effective February 1, and so on.
What Are Special Enrollment Periods?
SEPs let you make plan changes outside the standard windows when specific circumstances apply. It helps to understand how these windows actually work: an SEP is a fixed, continuous block of time triggered by your qualifying event — it doesn’t pause or split into pieces. Most commonly, that window is 2 full months starting from the month of the event (or the month you’re notified of it, if later), though the exact length varies by situation — some run 3 months, and a few are available anytime rather than on a countdown. If a second, separate qualifying event happens, that starts its own new SEP.
A few common examples:
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Losing job-based health coverage while Medicare-eligible — you can enroll in Original Medicare and a Medicare Supplement plan using an SEP. This one runs 2 full months after your coverage ends.
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Moving out of your plan’s service area — you’ll get an SEP to select new coverage where you live now, also a 2-month window from your move.
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A 5-star plan becomes available in your area — you can switch to it using the 5-star SEP, which runs on a fixed annual cycle from December 8 through November 30 (not year-round, and only once per cycle).
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Having a severe or disabling condition that a Chronic Care SNP in your area serves — this is a genuine “anytime” SEP, with a catch: you can join whenever you’re ready, but once you use it, that specific SEP is used up. It doesn’t reset or stay available on an ongoing basis.
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Being eligible for both Medicare and Medicaid, or for Extra Help — this one changed meaningfully in 2025, and it doesn’t follow the standard “fixed window” pattern at all. The old quarterly SEP is gone. In its place, eligible individuals now get a recurring, once-per-calendar-month opportunity to leave a Medicare Advantage plan and return to Original Medicare with a standalone Part D plan (or switch between standalone Part D plans) — though this SEP can’t be used to newly join a Medicare Advantage plan. A separate, more specific monthly SEP exists for enrolling in certain aligned Dual Special Needs Plans.
If more than one situation applies to you at once, each generally runs on its own timeline — it’s worth confirming your specific window rather than assuming, since the exact length and start date depend on which event triggered it.
Where Does Insurance Planning Fit In?
Knowing which window applies to your situation — and which one you might be about to miss — is exactly the kind of thing worth a quick check-in rather than guessing. Gilman Agency can help you figure out which enrollment period applies to you right now, and what your options actually are within it.
What To Do Next
If you’re not sure which enrollment period applies to your situation, or whether a recent life change has opened up an SEP you didn’t know about, reach out before you assume you have to wait until AEP.
Call (800) 927-9326, or email to talk through your situation. TTY: 711.
By Marc Gilman, Gilman Agency
Sources: Centers for Medicare & Medicaid Services (CMS); Medicare.gov; Justice in Aging, “Important Changes in 2025 to Special Enrollment Periods for Low-Income Medicare Enrollees.”
