TL;DR
Medicare has several enrollment windows. The Initial Enrollment Period (IEP) is your 7-month window around age 65. The Annual Enrollment Period (AEP, Oct 15–Dec 7) lets everyone review and change plans for the coming year. Special Enrollment Periods (SEPs) open when qualifying life events happen. Missing the right window can cost you.
Initial Enrollment Period (IEP)
Your IEP is the 7-month window around your 65th birthday — three months before, your birthday month, and three months after. This is when you first enroll in Parts A and B, and when you can pick up a Medicare Advantage or Medicare Supplement (Medigap) plan without health underwriting.
Annual Enrollment Period (AEP): Oct 15 – Dec 7
During AEP, anyone with Medicare can change Advantage plans, switch from Advantage back to Original Medicare, or join, drop, or change a Part D plan. Coverage changes take effect January 1. Because plans update benefits, networks, and drug formularies every year, this is when a yearly review pays off.
Medicare Advantage Open Enrollment (OEP): Jan 1 – Mar 31
If you're already in a Medicare Advantage plan on January 1, you get one additional switch — to a different Advantage plan or back to Original Medicare (with a Part D plan). You cannot use OEP to jump from Original Medicare into Advantage.
Special Enrollment Periods (SEPs)
SEPs open when qualifying events happen: moving out of your plan's service area, losing employer coverage, losing Medicaid, moving into or out of a nursing facility, and a handful of others. SEP length depends on the event.
The one to circle every year
AEP (October 15 – December 7). Even if you love your plan, spend 20 minutes reviewing next year's benefits, drug formulary, and provider network. A quiet plan change can quietly cost you thousands the following year.
