People talk about “open enrollment” as though Medicare had one. It has several, they are not interchangeable, and the one everybody hears about in October does not let you sign up for Part B at all. This guide separates them by what they actually permit.
Windows for signing up for Part A and Part B
These three decide when you can get into Medicare itself.
Initial Enrollment Period (IEP)
Seven months long: it starts three months before the month you turn 65 and ends three months after it. This is the window almost everyone should use, because missing it is what creates a lifelong Part B penalty.
General Enrollment Period (GEP)
January 1 to March 31 each year. This is the fallback if you missed your Initial Enrollment Period and do not qualify for a Special Enrollment Period. Coverage starts the month after you sign up, and a late enrollment penalty may apply.
Special Enrollment Period (SEP) for Part A and Part B
The most common one follows employer coverage: once you or your spouse stop working, or the job-based coverage ends, you have eight months to sign up for Part B without a penalty. The eight months start when the work or the coverage ends, even if you take COBRA instead — COBRA does not pause the clock.
Windows for choosing or changing a plan
These do not get you into Medicare. They let you pick, switch or drop the coverage that sits on top of it.
| Window | When | What you can do |
|---|---|---|
| Annual Enrollment Period | October 15 – December 7 | Join, drop or switch a Medicare Advantage plan; switch between Original Medicare and Advantage; join, drop or switch a Part D plan. Changes take effect January 1. |
| Medicare Advantage Open Enrollment | January 1 – March 31 | Only if you are already in a Medicare Advantage plan. Switch to another Advantage plan, or drop back to Original Medicare and add a Part D plan. |
| Special Enrollment Period | Varies | Triggered by a life event — moving, losing coverage, getting Medicaid or Extra Help. If you signed up for Part A or Part B through an SEP for an exceptional situation, you have two months to join a plan. |
Source: Medicare.gov, Joining a plan. The December 7 date is a receipt deadline, not a postmark deadline: the plan must have your request by then.
The distinction that catches people is the second row. The January to March window is only open to someone already in a Medicare Advantage plan. If you are in Original Medicare on January 1, it does nothing for you.
The Medigap window, which is not like the others
Medicare Supplement — Medigap — runs on its own clock and it is the least forgiving one in the system. The federal Medigap Open Enrollment Period is six months long and starts when you are 65 and enrolled in Part B. During it, an insurer must sell you a policy regardless of your health.
Outside it, and outside a handful of guaranteed-issue situations, an insurer can apply medical underwriting and decline you. Florida law does not add a birthday rule or an annual guaranteed-issue window on top of the federal minimum, so for most people here the six months really is the window.
JCKC does not sell Medigap. It is explained here because you cannot sensibly choose between Advantage and Medigap without knowing that one of the two doors closes permanently and the other does not.
The order these actually happen in
- Your Initial Enrollment Period opens three months before your 65th birthday month.
- If you are working past 65 with employer coverage, you may skip it and use the eight-month Special Enrollment Period later instead.
- The six-month Medigap window runs from the month you are both 65 and enrolled in Part B. It does not wait for you.
- Thereafter, October 15 to December 7 each year is when you review and change the plan on top.
- If you are in a Medicare Advantage plan and regret the choice, January 1 to March 31 gives you one correction.
Also on this site
Dated coverage of this topic, where the figures move year to year:
Frequently asked questions
Can I sign up for Part B in October during Annual Enrollment?
No. The October 15 to December 7 window only covers Medicare Advantage and Part D plans. To sign up for Part B outside your Initial Enrollment Period you need the General Enrollment Period of January 1 to March 31, or a Special Enrollment Period.
Does taking COBRA extend my time to enroll in Part B?
No, and this is a common and expensive mistake. The eight-month Special Enrollment Period starts when the employment or the job-based coverage ends, whichever comes first. Choosing COBRA does not restart or pause it.
If I miss December 7, when is my next chance?
If you are already in a Medicare Advantage plan, the Medicare Advantage Open Enrollment Period runs January 1 to March 31 and gives you one switch. If you are in Original Medicare, you generally wait for the next Annual Enrollment Period unless a life event gives you a Special Enrollment Period.
How long is the Medigap open enrollment window?
Six months, starting when you are 65 and enrolled in Part B. Inside it an insurer must sell to you regardless of health history. Outside it, in Florida, you can be underwritten and declined.
Sources
- When can I sign up for Medicare? — Medicare.gov · read 2026-10-19
- When does Medicare coverage start? — Medicare.gov · read 2026-10-19
- Joining a plan — Medicare.gov · read 2026-10-19
- When can I buy a Medigap policy? — Medicare.gov · read 2026-10-19
This guide explains published rules and figures, with the source for each one. It is not advice about your own situation.