- The Medicare Annual Enrollment Period (AEP) runs October 15 to December 7 every year; December 7 is the last day, and changes take effect January 1 — for 2026 coverage, that window ran October 15 to December 7, 2025.
- The Medicare Advantage Open Enrollment Period (MA-OEP), January 1 to March 31, lets people already in a Medicare Advantage plan make one switch; it is not the same as the AEP.
- Do not confuse Medicare's windows with the ACA Marketplace Open Enrollment Period, which runs about November 1 to January 15 in most states and is for under-65 marketplace coverage.
- Missing the AEP usually locks you into your current plan until the next year unless you qualify for a Special Enrollment Period.
- Late enrollment penalties for Part B and Part D can be permanent, so enroll on time or keep creditable coverage.
- Government figures such as the Part B premium and Part D base beneficiary premium change every year — confirm the current-year numbers at Medicare.gov.
Every year, millions of Medicare beneficiaries scramble to make plan changes before enrollment windows close, and missing the last day for Medicare open enrollment can lock you into a plan that costs more than it should or fails to cover the care you need. Medicare has several enrollment periods with different deadlines, each governing a different kind of change. Knowing exactly when each window closes — and what you are actually allowed to do before it does — is essential to avoiding penalties and controlling your healthcare spending. This article is general education, not insurance or legal advice; always confirm current dates and figures with Medicare.
Annual Enrollment Period: December 7 Is the Key Date
The Annual Enrollment Period (AEP), also called Medicare Fall Open Enrollment, runs from October 15 through December 7 every year. December 7 is the last day for Medicare open enrollment changes that take effect on January 1 of the following year. During the AEP you can switch Medicare Advantage plans, join or leave a Medicare Advantage plan, switch Part D prescription drug plans, or join a Part D plan for the first time. You can also move between Original Medicare and Medicare Advantage.
Any change you make by December 7 becomes effective January 1. For the 2026 plan year, this window ran October 15 through December 7, 2025, with new coverage starting January 1, 2026. If you miss the deadline, you are generally locked into your current plan until the next AEP unless you qualify for a Special Enrollment Period. That makes December 7 the single most consequential deadline for people who already have Medicare.
Do not wait until December 7 to decide. Processing delays, website traffic surges, and long phone hold times all spike in the final days of the AEP. Aim to finish by late November. Review your plan’s Annual Notice of Change (ANOC), which arrives in the fall and spells out how your premiums, drug coverage, and networks will change next year. For a fuller overview of this period, see our Medicare AEP guide.
Medicare Advantage Open Enrollment Period: March 31
The Medicare Advantage Open Enrollment Period (MA-OEP) runs from January 1 through March 31. This period is only for people who are already enrolled in a Medicare Advantage plan on January 1. During the MA-OEP you can switch to a different Medicare Advantage plan, or drop Medicare Advantage and return to Original Medicare and add a standalone Part D plan. You may make only one change during this window.
March 31 is the last day for MA-OEP changes. For the 2026 plan year, the MA-OEP runs January 1 through March 31, 2026. If you joined an Advantage plan during the AEP and are unhappy with it, the MA-OEP is your second chance to switch. Changes take effect the first day of the month after the plan receives your request. Note the key limit: the MA-OEP does not let someone in Original Medicare join a Medicare Advantage plan for the first time — it is only for those already in Advantage.
Don’t Confuse Medicare With the ACA Marketplace
People often mix up Medicare’s windows with the Affordable Care Act (ACA) Marketplace Open Enrollment Period, but they are entirely separate systems with separate deadlines. The ACA Marketplace Open Enrollment Period generally runs from about November 1 through January 15 in most states (some state-run exchanges use slightly different dates), and it is for people buying individual coverage — typically those under 65 who are not on Medicare. If you have Medicare, the Marketplace is not where you make changes; your windows are the AEP, the MA-OEP, and the other Medicare periods described here. Keep the two calendars straight so you act in the right place at the right time.
General Enrollment Period: March 31
The General Enrollment Period (GEP) runs from January 1 through March 31 each year and is for people who missed their Initial Enrollment Period for Medicare Parts A and B. If you failed to sign up for Part B when you first became eligible and do not have a Special Enrollment Period available, the GEP is your annual chance to enroll. Under current rules, coverage begins the first day of the month after you sign up.
The last day for the GEP is March 31. Be aware that late enrollment penalties usually apply. The Part B penalty is 10 percent of the standard premium for each full 12-month period you were eligible but did not enroll, and it is generally permanent. For example, the standard Part B premium for 2026 is $202.90 per month (higher-income beneficiaries pay more), so a two-year delay would add roughly 20 percent — about $40 per month at that premium — on top of your Part B premium for as long as you have Part B. Because these figures change every year, confirm the current premium at Medicare.gov. For details on all enrollment timelines, see our Medicare enrollment guide.
Initial Enrollment Period: Your Personal Deadline
Your Initial Enrollment Period (IEP) is a seven-month window centered on your 65th birthday month: it begins three months before the month you turn 65 and ends three months after. For example, if you turn 65 in April, your IEP ends July 31. Missing this deadline can mean waiting for the next GEP to enroll, plus facing late enrollment penalties and a possible gap in coverage.
If you are approaching 65 and still working with active employer coverage from an employer with 20 or more employees, you can usually delay Part B enrollment without penalty. Your Special Enrollment Period begins when the employer coverage or the employment ends, giving you eight months to enroll in Part B without a penalty. Retiree coverage and COBRA do not count as active employer coverage for this purpose, so do not rely on them to delay Part B.
Medigap Open Enrollment Period: Six Months After Part B Starts
Your Medigap Open Enrollment Period begins the first day of the month you are both 65 or older and enrolled in Part B, and it lasts six months. The last day is exactly six months after it starts. During this one-time window, Medigap insurers must sell you any plan they offer at the standard rate regardless of your health — this is your guaranteed-issue right.
Missing the Medigap OEP does not bar you from buying Medigap later, but it removes that guaranteed-issue protection. Afterward, in most states, insurers can medically underwrite your application and may deny coverage or charge more based on pre-existing conditions. A few states, such as California with its “birthday rule,” offer additional annual protections — rules vary, so check your state. For plan comparisons, visit our compare Medicare supplement plans guide.
Part D Enrollment: Avoiding the Late Penalty
You should enroll in Part D during your IEP or within 63 days of losing creditable drug coverage. If you go 63 or more consecutive days without creditable drug coverage, a late enrollment penalty can apply. The penalty is roughly 1 percent of the “national base beneficiary premium” for each full month you went without coverage, and it is generally permanent. That base premium is set by CMS each year (it was $36.78 for 2025), so the exact dollar penalty shifts annually — check the current figure at Medicare.gov.
As a rough illustration using a base premium in that range, going without creditable coverage for 12 months could add several dollars per month to your Part D premium for as long as you have Part D — a small monthly amount that adds up to hundreds of dollars over the years. Separately, thanks to recent law, Part D now includes an annual out-of-pocket cap on covered drugs (it was $2,000 in 2025 and is indexed to rise in later years — verify the current-year amount), which can significantly help people with high drug costs. Learn more about drug coverage in our Part D donut hole article.
Special Enrollment Periods: Year-Round Exceptions
Special Enrollment Periods (SEPs) can occur at any time of year when triggered by a qualifying life event. Common SEP triggers include losing employer or union health coverage, moving out of your plan’s service area, gaining or losing Medicaid eligibility, qualifying for the Part D Extra Help program, and being affected by a natural disaster or other emergency declared by federal or state authorities.
Each SEP has its own duration and deadline. The employer-coverage SEP lasts eight months from the date the coverage or employment ends. Move-related SEPs typically allow up to two months after the move. Always confirm the exact deadline for your specific SEP with Medicare at 1-800-MEDICARE (1-800-633-4227) or your State Health Insurance Assistance Program (SHIP) counselor. See our Medicare Extra Help guide for information on low-income assistance SEPs.
What Happens If You Miss a Deadline
The consequences of missing a Medicare deadline vary by period. Missing the AEP usually means staying in your current plan for another year, which may mean higher costs or reduced coverage. Missing the IEP can result in coverage gaps and permanent premium penalties. Missing the Medigap OEP means losing guaranteed-issue rights and potentially being denied or upcharged for a supplement in states without extra protections.
In most cases you cannot retroactively enroll or undo a missed deadline. The main exception is if CMS or your plan made an error that prevented you from enrolling on time, in which case you may be able to request an equitable relief exception. Contact 1-800-MEDICARE to report an enrollment processing problem and ask what options you have.
Frequently Asked Questions
Is December 7 the last day to change my Medicare plan?
December 7 is the last day of the Annual Enrollment Period, the primary window for switching Medicare Advantage and Part D plans. However, Medicare Advantage enrollees get an additional window (the MA-OEP) from January 1 through March 31 to make one more change. Other enrollment periods have their own separate deadlines.
Can I change my Medigap plan during the AEP?
No. The AEP applies only to Medicare Advantage and Part D. Medigap enrollment and switching follow separate rules and are not tied to the October 15 through December 7 window. You can apply to change Medigap plans at other times, but outside guaranteed-issue situations you may face medical underwriting.
What if I forgot to sign up for Medicare at 65?
If you missed your IEP and do not have qualifying active employer coverage, you can enroll during the GEP from January 1 through March 31, with coverage starting the following month under current rules. You will likely face a permanent Part B late enrollment penalty of 10 percent for each full year you delayed.
Can I drop Medicare Advantage after the AEP?
Yes, during the MA-OEP from January 1 through March 31. You can leave your Advantage plan, return to Original Medicare, and add a Part D plan. After March 31 you are generally locked in unless you have a qualifying SEP. For help deciding between Advantage and supplement plans, see our Medicare Advantage vs supplement comparison.
How is the Medicare AEP different from ACA Open Enrollment?
They are separate programs. The Medicare AEP (October 15 to December 7) is for people with Medicare. The ACA Marketplace Open Enrollment Period (about November 1 to January 15 in most states) is for people buying individual coverage, usually under 65. If you have Medicare, you use the Medicare windows, not the Marketplace.
Mark Your Calendar and Act Early
The single most useful thing you can do is put every relevant deadline on your calendar and start reviewing your options at least six weeks before each window opens. For the AEP, begin comparing plans in early October when insurers publish their Annual Notices of Change. For your IEP or Medigap OEP, start gathering quotes about three months before your 65th birthday. Use the official plan finder at Medicare.gov, contact your SHIP counselor for free unbiased help, or work with a licensed independent broker to compare available options. Missing a deadline is almost always more expensive than the time it takes to review your choices. For comprehensive Medicare planning, explore our healthcare policy guide and our Medigap plans overview.
This article is general education, not insurance or legal advice. Medicare dates are stable from year to year, but premiums, the Part D base beneficiary premium, penalties, and program rules change annually. Confirm current-year figures and your personal deadlines at Medicare.gov or by calling 1-800-MEDICARE (1-800-633-4227), and consider free help from your State Health Insurance Assistance Program (SHIP).
Sources
- Medicare.gov — Joining a health or drug plan; when you can join, switch, or drop coverage; 2026 Medicare costs
- Centers for Medicare & Medicaid Services (CMS) — Medicare enrollment periods; Part D late enrollment penalty; national base beneficiary premium
- HealthCare.gov — ACA Marketplace Open Enrollment dates (for contrast)
- State Health Insurance Assistance Programs (SHIP) — Free local Medicare counseling