“Open enrollment” isn't one universal calendar — it's several separate windows depending on where your coverage comes from, and missing the wrong one has real consequences.
| Coverage type | Enrollment window | Note |
|---|---|---|
| ACA Marketplace | Nov 1 – Jan 15 | Federal deadline; a few state-run exchanges extend it further. |
| Medicare Annual Election Period | Oct 15 – Dec 7 | Switch Medicare Advantage or Part D plans, or move between MA and Original Medicare. |
| Medicare Advantage Open Enrollment | Jan 1 – Mar 31 | For those already enrolled in an MA plan — one plan switch allowed. |
| Employer group plans | Set by the employer | Commonly falls in Q4, but varies — check with HR. |
What happens if you miss the ACA marketplace window
Outside of Nov 1 – Jan 15, you generally can't enroll in or change a marketplace plan unless you qualify for a Special Enrollment Period (SEP)— triggered by a qualifying life event like losing other coverage, getting married, having or adopting a child, or moving to a new coverage area. An SEP typically gives you 60 days from the event to enroll. Without a qualifying event, missing the window usually means waiting until the next one — there's no general exception for “I forgot.”
What happens if you miss a Medicare window
Missing the Annual Election Period doesn't leave you without coverage — your existing Medicare Advantage, Part D, or Original Medicare enrollment simply continues as-is for another year. What you lose is the chance to switch plans until the next AEP (or the narrower MA-only window in Q1). Missing your Initial Enrollment Period around your 65th birthday is the one that can carry a real penalty — a late-enrollment penalty on Part B or Part D that can apply for as long as you have that coverage, unless you qualify for an exception (most commonly, having other creditable coverage through an employer).
What happens if you miss an employer's window
This is the least standardized of the three — some employers offer a brief grace period, most don't. The same qualifying-life-event logic that triggers an ACA marketplace SEP usually applies here too (most group plans are required to allow it), but outside of that, missing an employer's deadline commonly means waiting for the next plan year.
Check the specific dates that apply to you: These are the standard federal windows — some states run their own ACA exchanges with extended deadlines, and employer dates always vary. Confirm the exact dates with your marketplace, your Medicare plan, or your HR department rather than assuming the general calendar above applies exactly to your situation.
Deciding what to buy once you know your window? See Bronze, Silver, Gold, Platinum: what you actually pay.