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Medicare Enrollment Periods Explained: Every Window and Deadline

A plain-English guide to Medicare's enrollment windows — IEP, General Enrollment, MA Open Enrollment, fall Open Enrollment, SEPs, and late penalties.

Last reviewed: July 2026

Why enrollment windows matter

Medicare is not a sign-up-anytime program. Nearly every action — getting Part A and Part B for the first time, joining a Medicare Advantage or Part D drug plan, or switching between them — is tied to a specific calendar window. Miss the right one and the consequences range from a gap in coverage to a premium penalty that lasts for life. This guide walks through each enrollment period, who it applies to, and how the late-enrollment penalties are calculated. (If you are still sorting out what Parts A, B, C, and D actually cover, start with our guide to the parts of Medicare.)

The Initial Enrollment Period (IEP)

The Initial Enrollment Period is your first chance to sign up, built around your 65th birthday. It lasts seven months: the three months before the month you turn 65, your birthday month, and the three months after.

People already receiving Social Security benefits at 65 are typically enrolled in Parts A and B automatically. Everyone else has to apply, usually through the Social Security Administration at ssa.gov. The IEP is also the first window to join a Medicare Advantage plan or a Part D drug plan.

The General Enrollment Period (January 1 – March 31)

If you missed your IEP and do not qualify for a Special Enrollment Period, the General Enrollment Period (GEP) is the fallback for signing up for Part A and/or Part B. It runs January 1 through March 31 each year, and coverage begins the first of the month after you enroll — another improvement that took effect in 2023, replacing the old rule that made everyone wait until July 1. People who enroll in Part B during the GEP also get an opportunity to join a Part D plan so drug coverage can line up with their new Part B. The catch: using the GEP usually means the Part B late-enrollment penalty applies if you went 12 or more months without coverage you could have had — the penalty is based on each full 12-month period of delay.

Fall Open Enrollment (October 15 – December 7)

Medicare's Open Enrollment Period — sometimes called the Annual Enrollment Period or AEP — is the once-a-year window when anyone with Medicare can rework their coverage for the following year. Between October 15 and December 7 you can:

Changes take effect January 1. This is the window most people use to react to their plan's Annual Notice of Change, which arrives each fall and spells out next year's premiums, drug lists, and provider networks. Comparing options is free at medicare.gov's Plan Finder or by calling 1-800-MEDICARE. For a neutral look at the trade-offs between the two coverage routes, see our Medicare Advantage vs. Original Medicare guide.

Medicare Advantage Open Enrollment (January 1 – March 31)

This window is easy to confuse with the GEP because the dates match, but it serves a different group: people who are already in a Medicare Advantage plan on January 1. During it, you may make one change — switch to a different Medicare Advantage plan, or drop Medicare Advantage and return to Original Medicare (with the option to join a Part D plan). Changes take effect the first of the following month.

Two limits are worth underlining. You cannot use this period to join Medicare Advantage for the first time or to switch between stand-alone Part D plans. And returning to Original Medicare does not guarantee you can buy a Medigap policy — outside your one-time Medigap open enrollment window, insurers in most states can decline you or charge more based on health. Our Medigap basics guide covers those rules. A separate version of this window also lets people who joined a Medicare Advantage plan when they first got Medicare make one change during their first three months of having Medicare Part A and Part B.

Special Enrollment Periods (SEPs)

Special Enrollment Periods let people enroll or change coverage outside the standard windows when specific life events occur. The most important one applies to people still working at 65: if you (or your spouse) have group health coverage from a current employer, you can delay Part B without penalty and then sign up during an eight-month SEP that begins the month after the employment or the coverage ends, whichever comes first. Two common traps: COBRA and retiree coverage do not count as current employer coverage, so they do not extend the eight-month clock or protect you from the Part B penalty.

Other SEPs cover situations such as moving out of your plan's service area, losing Medicaid or Extra Help, your plan leaving Medicare, entering or leaving a nursing home, release from incarceration, and being affected by a declared disaster or emergency. Most Medicare Advantage and Part D SEPs give you about two months to act, though the details vary by circumstance — the full list is on medicare.gov.

Late-enrollment penalties, in plain numbers

Three separate penalties exist, and they work differently.

Employer drug coverage often counts as creditable; your plan is required to tell you each year whether it is. Keeping those notices makes it much easier to contest a penalty later.

Where to get help

Which window applies to you — and whether delaying makes sense in your situation — depends on your work status, other coverage, income, and health needs. No article can settle that for you, and this one doesn't try. For free, personalized, unbiased help, contact your State Health Insurance Assistance Program (SHIP) at shiphelp.org, call 1-800-MEDICARE (available 24 hours a day, 7 days a week, except some federal holidays), or use the tools at medicare.gov.

Sources

This guide is for general education only and is not medical, legal, insurance, or financial advice. For decisions about your own coverage, use official sources or free help from your SHIP counselor.