If you have ever tried to buy health insurance outside of a specific window, you may have discovered that you cannot simply sign up whenever you want. Health insurance operates on a system of enrollment periods, and understanding these timelines is essential if you want to avoid gaps in coverage or, worse, going an entire year without insurance.
Different types of insurance have different enrollment windows. The ACA marketplace, employer plans, Medicare, FEHB, TRICARE, and Medicaid all operate on their own schedules. This guide covers the enrollment rules for each, including key dates, what to do if you miss the window, and how to prepare.
What Is Open Enrollment?
Open enrollment is a designated period each year when you can sign up for a new health insurance plan, switch your existing plan, or make changes to your coverage. Outside of this window, you generally cannot enroll in or change your health insurance unless you qualify for a Special Enrollment Period.
The concept exists to keep the insurance market stable. If people could sign up for coverage only when they got sick and drop it when they were healthy, premiums would skyrocket for everyone.
ACA Marketplace Open Enrollment
The ACA marketplace has a set annual open enrollment period. For coverage beginning in 2027, the federal marketplace open enrollment runs from November 1, 2026 through January 15, 2027. To have coverage start January 1, 2027, you must enroll by December 15, 2026. Enrollments completed between December 16 and January 15 generally take effect February 1, which means a two-week delay in shopping can translate into a month without coverage.
Several states run their own marketplaces rather than using HealthCare.gov, and their deadlines can differ from the federal dates in either direction. If you live in a state with its own exchange, confirm your dates on the state site rather than assuming the federal calendar applies.
Two cautions matter more than usual this cycle. First, if you do nothing, most marketplaces will automatically re-enroll you in your current plan or its closest equivalent. Auto-renewal prevents a coverage gap, but it silently accepts whatever happened to your premium, your network, and your subsidy, and all three change every year. Second, the enhanced premium subsidies that existed through 2025 have expired. In 2026 the subsidy cliff at 400% of the federal poverty level is back, so households above roughly $62,600 for a single person receive no premium tax credit at all. If your subsidy looked generous last time you checked, re-run the numbers rather than trusting memory. And estimate your income honestly: premium tax credits are reconciled on your tax return, and an overstated subsidy becomes a repayment.
Employer-Sponsored Open Enrollment
If you get health insurance through your job, your employer sets its own open enrollment period, typically in the fall for coverage starting January 1. During this window, you can enroll in your employer plan, switch between plan options, add or remove dependents, and enroll in or change your FSA, HSA, dental, or vision elections.
Read the enrollment materials closely enough to know whether your employer runs passive or active enrollment. In a passive enrollment, doing nothing rolls your current elections forward. In an active enrollment, doing nothing can leave you with no coverage at all. One election never rolls forward either way: health FSA contributions must be chosen fresh each year. If you are married and both spouses have employer offers, open enrollment is the natural moment to compare the two plans side by side, since both sets of rates and designs reset at the same time. Note also that having an affordable employer offer, defined for 2026 as employee premiums of no more than 9.96% of household income, generally blocks you from marketplace subsidies, so for most employees the real choice is among the employer options, not between employer and marketplace.
Medicare Enrollment Periods
Medicare has not one enrollment period but several, and confusing them is one of the most common and expensive mistakes in American health insurance.
The Initial Enrollment Period (IEP) is a seven-month window around your 65th birthday: the three months before your birthday month, the month itself, and the three months after. This is when most people first sign up for Parts A and B, and the deadline has teeth. Missing Part B without other qualifying coverage triggers a late enrollment penalty of 10% of the premium for each full 12-month period you delayed, and you pay it for as long as you have Part B. Part D, the drug benefit, has its own smaller but likewise permanent penalty that accrues monthly.
The Annual Enrollment Period (AEP) runs from October 15 through December 7 each year, with changes effective January 1. This is when anyone on Medicare can switch between Original Medicare and Medicare Advantage, change Medicare Advantage plans, or change Part D drug plans. Each September, your plan mails an Annual Notice of Change describing what your premiums, benefits, and drug coverage will look like next year; reading it is the single highest-value fifteen minutes in the Medicare calendar.
The Medicare Advantage Open Enrollment Period (MA-OEP) runs from January 1 through March 31 and is widely misunderstood. It is only for people already enrolled in a Medicare Advantage plan, and it allows exactly one change: switching to a different Medicare Advantage plan, or dropping Medicare Advantage for Original Medicare plus a Part D plan. It does not allow someone on Original Medicare to join Medicare Advantage. Think of it as an escape hatch for people who discover in January that their new plan does not include their doctors.
One more trap for people working past 65: employer coverage from active employment lets you delay Part B safely, and you get a special enrollment period of eight months once that employment ends. COBRA does not count for this purpose. Retiring at 67, taking 18 months of COBRA, and only then enrolling in Part B is a recipe for a permanent penalty.
Medicaid and CHIP: No Window at All
Medicaid and CHIP are the exception to everything above: you can apply any day of the year, and coverage can begin quickly once you are found eligible. Eligibility is based on current monthly income, which makes Medicaid the natural first stop after a sudden income loss, even for people who have never been close to qualifying before. The flip side of year-round enrollment is year-round renewal: states periodically redetermine eligibility, and the most common way people lose Medicaid is not earning too much but failing to return a renewal packet. If you are on Medicaid, keep your address current with the state and answer renewal mail promptly. Losing Medicaid is itself a qualifying event, with 60 days to enroll in a marketplace plan and, unusually, 60 days rather than 30 to join an employer plan.
FEHB and TRICARE Enrollment Seasons
Federal civilian employees, retirees, and their families enroll through the Federal Employees Health Benefits program, which holds its annual Open Season from mid-November through mid-December each year, with changes effective in early January. Outside Open Season, FEHB allows changes only after qualifying life events, on rules broadly parallel to the private-sector ones. Federal employees who lose coverage have their own continuation program, Temporary Continuation of Coverage, rather than COBRA.
TRICARE, which covers military service members, retirees, and their families, runs its own Open Season over roughly the same mid-November to mid-December window. It applies to TRICARE Prime and TRICARE Select: you can switch between them or change enrollment for family members, with changes effective January 1. Outside Open Season, TRICARE also uses qualifying life events, and a QLE for one family member opens a window for the whole family. Active duty service members are enrolled in Prime automatically and do not need to act.
What If You Miss Open Enrollment?
Missing the window does not always mean waiting a year. Qualifying life events, such as losing other coverage, marriage, birth, or a move, open special enrollment periods throughout the year. Medicaid and CHIP accept applications continuously. Members of federally recognized tribes can enroll in or change marketplace plans monthly. But if none of those apply, the honest answer is that you wait, which is exactly why the fall deadlines deserve a calendar entry every year.
How to Prepare
Whichever system you are in, the preparation is similar. Before your window opens, list the doctors you cannot lose and the prescriptions you take, then check both against each plan you are considering rather than assuming networks and formularies stayed the same. Estimate next year income carefully if you are subsidy-eligible. Read the change notices your current plan sends in the fall instead of recycling them. And put the dates that apply to you in writing: November 1 to January 15 for the marketplace, October 15 to December 7 for Medicare, mid-November to mid-December for FEHB and TRICARE, and whatever weeks your employer announces. Enrollment season rewards the people who show up to it on time and with their paperwork in order, and it is indifferent to everyone else.