The four fixed dates
Most of the calendar year has no enrollment activity at all. Four dates carry nearly all of it, and if you remember only these you will be ahead of most people:
- October 15. Medicare AEP opens.
- November 1. ACA marketplace Open Enrollment opens.
- December 7. Medicare AEP closes. Hard stop.
- December 15. Marketplace cutoff for coverage that starts January 1.
Everything else is either tied to your birthday, tied to something that happened in your life, or a cleanup window in the first quarter for people who missed the fall.
Medicare windows
| Window | Dates | Who it is for | What you can do |
|---|---|---|---|
| Initial Enrollment Period (IEP) | 7 months around your 65th birthday | Anyone becoming eligible for Medicare | Enroll in Part A and Part B, and choose Medicare Advantage or a drug plan |
| Medigap Open Enrollment | 6 months from the month you are 65 and enrolled in Part B | Anyone newly enrolled in Part B at 65 or older | Buy any Medicare Supplement policy with no medical underwriting |
| Annual Enrollment Period (AEP) | October 15 to December 7 | Anyone with Medicare | Join, switch, or drop a Medicare Advantage or Part D plan |
| Medicare Advantage Open Enrollment (MA OEP) | January 1 to March 31 | People already in a Medicare Advantage plan | One change: another Advantage plan, or back to Original Medicare with a drug plan |
| General Enrollment Period (GEP) | January 1 to March 31 | People who missed their IEP with no Special Enrollment Period | Enroll in Part A and Part B; coverage starts the month after you sign up |
| Special Enrollment Periods (SEP) | Varies by circumstance | People who move, lose coverage, or change Medicaid or Extra Help status | Enroll or switch outside the regular windows, usually without penalty |
Medigap is not on this calendar, and that surprises people. Medicare Supplement policies are regulated by the state, not by the federal enrollment periods. You may apply any month of the year. What changes is whether the carrier can say no. Inside your six-month Medigap open enrollment, it cannot. Outside it, in Arkansas, it can, because this state has no birthday rule and no annual guaranteed issue window.
ACA marketplace windows
| Window | Dates | Who it is for | What you can do |
|---|---|---|---|
| Open Enrollment Period (OEP) | November 1 to January 15 | Anyone buying individual coverage who is not on Medicare | Enroll in or change a HealthCare.gov plan. December 15 is the cutoff for January 1 coverage |
| Special Enrollment Period (SEP) | 60 days from a qualifying life event | People who lose coverage, marry, have a child, or move | Enroll in or change a marketplace plan outside Open Enrollment |
Arkansas uses the federal marketplace at HealthCare.gov and has no state-run exchange, so the federal dates apply here with no state variation. That also means Arkansas does not get the extended windows some state exchanges set for themselves.
One caution on the marketplace dates specifically. A federal rule would cut the window short at December 15 beginning with the 2027 plan year. A court vacated that provision in June 2026 and the appeal has not been decided, so HealthCare.gov currently still publishes January 15 as the end date. This page is checked against HealthCare.gov each year, but if you are reading it close to a deadline, confirm the date on the source before you rely on it.
How the year actually runs
Laid out in order, the year has a shape that is easier to hold onto than a list of periods.
| When | What is happening |
|---|---|
| January to March | Cleanup. Medicare Advantage Open Enrollment gives one change to people already in an Advantage plan. General Enrollment catches people who missed Part B entirely. The marketplace closes on January 15. |
| April to August | Quiet. Nothing opens on the calendar. Enrollment in this stretch happens through Special Enrollment Periods, or because someone is turning 65. |
| September | Annual Notice of Change letters arrive from Medicare Advantage and Part D plans, describing what changes on January 1. This is preparation month. |
| October to December | The heavy season. Medicare AEP opens October 15 and closes December 7. Marketplace Open Enrollment opens November 1. December 15 is the cutoff for January 1 marketplace coverage. |
Windows that follow you rather than the calendar
Three of the seven have nothing to do with the date. Your Initial Enrollment Period is set by your birthday. Your Medigap open enrollment is set by the month your Part B starts. Special Enrollment Periods are set by something happening to you: a move, a job ending, a change in Medicaid or Extra Help eligibility, a plan pulling out of your county.
These are the windows people miss most often, because there is no advertising campaign reminding you that yours has opened. If your circumstances change, the clock has probably already started. Most Medicare SEPs run two months from the triggering event, the employer coverage SEP for Part B runs eight, and the marketplace gives you sixty days. None of them wait for you to notice.
What to do with this page
Bookmark it and check it in September, before the mail and the television advertising start. If you can answer two questions, which window applies to me and what is its deadline, the rest of the process is ordinary paperwork.
If you cannot answer them, that is a short phone call. Lancaster Cook is an independent agent in Little Rock, AHIP certified for Medicare and FFM certified for the marketplace, Arkansas license #8021079. There is no cost for the conversation and no cost for the enrollment.