Medicare Enrollment Timeline Explained (AEP, OEP, and Options)

Medicare doesn’t operate on a single deadline. It runs on several, and each one means something different. If you’ve ever wondered whether you’re in the right window to make a change, this is the page that puts every one of them on a single timeline.

The Big Picture: Medicare’s Enrollment Windows at a Glance

Window	Who It’s For	When It Runs	What It Allows
Annual Enrollment Period (AEP)	Anyone with Medicare	October 15 – December 7	Switch plans or drug coverage, effective January 1
Medicare Advantage Open Enrollment Period (OEP)	People already in a Medicare Advantage plan	January 1 – March 31	One plan change, or a return to Original Medicare
Special Enrollment Periods (SEP)	People with a qualifying life change	Varies by event	Enroll or change coverage outside the normal calendar
Initial Enrollment Period (IEP)	People newly eligible for Medicare	7 months around your 65th birthday	One-time sign-up for Medicare Parts A and B

This is the single view none of our other Medicare posts give you in one place. Below, we’ll walk through each window in a bit more detail.

Annual Enrollment Period (AEP): October 15 to December 7

AEP is open to anyone with Medicare. Changes made during this window take effect January 1, and it’s the main window each year for switching plans or drug coverage. For the full breakdown, see our guide to Understanding the Medicare Annual Enrollment Period (AEP).

Medicare Advantage Open Enrollment Period (OEP): January 1 to March 31

OEP is open only to people already enrolled in a Medicare Advantage plan. It allows one plan change or a return to Original Medicare, but it does not allow you to switch from Original Medicare to Medicare Advantage. For the full mechanics, see What Is the Medicare Advantage Open Enrollment Period (OEP), and How Does It Work?

Special Enrollment Periods (SEP): When Life Changes Outside the Calendar

This is the window most people don’t know exists until they need it. A Special Enrollment Period opens when something in your life changes outside the normal calendar, and several common events can trigger one:

• Moving out of your plan’s service area
• Losing employer or union coverage
• Moving into or out of a skilled nursing facility
• Your plan’s contract with Medicare is ending
• Becoming newly eligible for both Medicare and Medicaid
• When there is a state of emergency in effect

SEP timing and length vary by the specific qualifying event, so confirm your exact window with Medicare or a licensed advisor rather than assuming a standard length.

Initial Enrollment Period (IEP): Your One-Time Window

If you’re approaching Medicare eligibility for the first time, you also have your own one-time window: a 7-month period built around your 65th birthday. For the full step-by-step breakdown of deadlines and penalties, see our Turning 65 Checklist.

Which Window Applies to You Right Now?

• Turning 65 this year? You’re in your Initial Enrollment Period.
• Want to compare plans this fall? That’s AEP, October 15–December 7.
• Already in a Medicare Advantage plan and had a change of heart in February? That’s OEP.
• Just moved, or lost other coverage? Check whether you qualify for a Special Enrollment Period.

Mistakes That Cost People Their Chance to Switch

• Confusing AEP with OEP, since they run at different times and allow different changes
• Assuming OEP lets you move from Original Medicare into Medicare Advantage (it doesn’t)
• Missing a Special Enrollment Period’s documentation deadline
• Assuming a change takes effect immediately without confirming the actual effective date

Put It on Your Calendar

October 15 to December 7: AEP, review and switch plans. January 1 to March 31: OEP, one Medicare Advantage change allowed. All year: watch for life changes that could open a Special Enrollment Period. Around your 65th birthday: your one-time Initial Enrollment Period.

Talk to Your Care Team Before You Decide

Before switching plans, loop in your primary care team. They can help confirm a new plan still covers your doctors. Schedule a visit with Palm Medical Centers to talk through your coverage before your next enrollment decision.

Frequently Asked Questions

What’s the difference between AEP and OEP?

AEP (October 15–December 7) is open to anyone with Medicare and allows a full plan or drug coverage switch. OEP (January 1–March 31) is open only to current Medicare Advantage members and allows one change.

Can I switch from Original Medicare to Medicare Advantage during OEP?

No. OEP only allows changes for people already enrolled in a Medicare Advantage plan.

What happens if I miss AEP?

Outside of a qualifying Special Enrollment Period, you’ll generally need to wait for the next AEP or, if eligible, use OEP to make one Medicare Advantage change.

Do I need to do anything during AEP if I’m happy with my current plan?

Not necessarily, but it’s still worth a quick annual review, since plan costs and coverage can change year to year even if you don’t.

This article is educational, not personalized insurance advice. Enrollment dates and rules are set by Medicare and may be updated. Confirm current details at Medicare.gov, call 1-800-MEDICARE, or talk with a licensed SHIP counselor.