When is open enrollment for Medicare Supplement plans in Arizona?

Medicare Supplement open enrollment is not a season. It is a one-time six-month window that starts the first day of the month you are both 65 and enrolled in Part B, and it is different for every person. If your Part B starts March 1, 2027, yours runs through August 31, 2027. The October 15 to December 7 period on every mailer has nothing to do with it. Inside those six months, any Medicare Supplement sold in Arizona has to take you with no health questions. After them, an Arizona insurer can ask, and can say no.

I am writing this five days before the fall enrollment period opens, because this is the week the two get confused. Everyone in Vistancia and Trilogy is hearing “open enrollment” from the television, and the people a few months short of 65 reasonably assume that is their deadline for everything. For a Medicare Advantage or Part D plan it is a real date. For a Supplement it is the wrong calendar entirely, and the right one is printed nowhere except on your own Medicare card, in the line that says when Part B began.

The dates if you turn 65 in early 2027

The rule from Medicare.gov is one sentence: the window is “the 6-month period that starts the first day of the month you’re 65 or older and signed up for Part B.” So the dates follow your Part B start date, and your Part B start date follows when you sign up.

Sources: Medicare.gov, "When to buy a Medigap policy" and "When does Medicare coverage start?" The table assumes you sign up in the three months before your birthday month, so Part B begins the first of that month. Sign up later and Part B starts later, and the six months start with it.

Three things about that table.

The window moves with Part B, not with your birthday. If you sign up for Medicare during your birthday month or the three months after it, Part B starts the first of the following month, and the six months begin then. Nothing is lost by that. The window is always six full months from whenever Part B takes effect at 65 or older.

A birthday on the first of the month moves everything one month earlier, the same way it moves your Medicare start date. Do you automatically get Medicare when you turn 65? has that arithmetic, and the enrollment timeline tool builds both windows from your own birth date.

Aim for the policy to start the same day Part B does. You do not have to wait for the window to open before you choose. Ask the company how far ahead it will take an application, and line the Supplement up to begin on your Part B date so there is no month where Original Medicare stands alone with its $1,736 hospital deductible and its uncapped 20%.

What the six months actually guarantee

The federal guide, Choosing a Medigap Policy, lists what an insurer cannot do during your window. It cannot refuse to sell you any Medigap policy it offers. It cannot use medical underwriting to decide whether to accept you. It cannot charge you more because of health problems. That is the whole value of the period: a heart history, a cancer diagnosis five years back or a knee replacement on the calendar are all irrelevant to the application.

One thing the window does not fully remove is the pre-existing condition waiting period. Even inside open enrollment, an insurer may decline to pay its share for a pre-existing condition for up to six months, and only if that condition was treated or diagnosed in the six months before the policy starts. Original Medicare still pays its part during that time. The exception is the one that covers nearly everyone I meet: if you had at least six months of continuous creditable coverage beforehand, with no break longer than 63 days, the insurer cannot make you wait at all. An employer plan you held until the month before Medicare counts. Ask the company the question directly, “do you have a waiting period for pre-existing conditions,” and get the answer before you sign.

What changes on the first day of month seven

Outside the window, the same federal guide says three things can happen: you may pay more, fewer policies may be available, and “there’s no guarantee that an insurance company will sell you a Medigap policy if you don’t meet their medical underwriting requirements.”

In Arizona that sentence is the whole story, because the state has not added anything to it. There is no birthday rule and no annual window here. What remains are the federal guaranteed issue rights, which open only after specific events, such as a Medicare Advantage plan leaving your area or certain employer coverage ending, and which cover a narrower list of plan letters than open enrollment does. They are real protections, and they are not a plan. Can I change my Medicare Supplement plan anytime in Arizona? covers what an underwritten application looks like and why people from California, Nevada and Idaho are caught off guard by it.

To be clear about what this is and is not: nobody is penalized for skipping a Supplement. There is no late fee and no mark on your record. Plenty of people choose Medicare Advantage at 65 on purpose and are well served by it. The point is narrower. The six months are the only time the choice between the two roads is fully yours, and after that, part of the decision belongs to an underwriter.

Three ways the window gets used up without anyone noticing

These are the cases I actually see in north Peoria, and none of them involve anyone being careless.

Taking Part B at 65 while still on a big employer plan. This one is common among people who are still working, or whose spouse is, at an employer with 20 or more employees. They enroll in Part B at 65 because it feels like the responsible thing to do, and keep the group plan as their real coverage. Medicare.gov’s wording is that the Medigap window “starts once you sign up for Part B, even if you sign up for Part B while you still have employer coverage.” Put dates on it. Part B starts March 1, 2027, the window closes August 31, 2027, and retirement comes in December 2028. By the time a Supplement would be useful, the six months ended sixteen months earlier, and the Part B premium of $202.90 a month, plus any IRMAA on top, was paid the whole time for coverage that sat behind the group plan. The federal guide says it plainly: with coverage from current employment, “you may want to wait to sign up for Part B,” so that the window starts “when you’re ready to take advantage of it.” Whether you can wait depends on the employer’s size, and I worked through that test in Do I need to sign up for Medicare at 65 if I’m still working?

Starting on Medicare Advantage and assuming the Supplement will still be there. Choosing an Advantage plan at 65 does not stop the six-month clock. It runs in the background and expires. There is one safety net, called a trial right. If you joined an Advantage plan when you were first eligible at 65, and within the first year you decide to move to Original Medicare, you can buy any Medigap policy sold in Arizona without health questions, applying from 60 days before your plan coverage ends to 63 days after. That gives a new enrollee roughly twelve months to find out whether the network works with the Arrowhead and Sun City specialists they actually use. After that first year, the trial right is gone too, and Can I switch from Medicare Advantage to Medigap in Arizona? describes what is left.

Arriving mid-window from another state. A good share of 85383 turned 65 somewhere else and moved here within the year. The window is federal, so it came with you, and so did its end date. If your Part B began in January in Illinois and you closed on a house in Blackstone in April, you have until June 30 to buy any Supplement sold in Arizona with no health questions, and after that you are an Arizona applicant under Arizona rules. People from states with a yearly switching right tend to assume there will be another chance. Here there is not.

Why this is a bigger question in Peoria than the mailers suggest

Maricopa County has one of the densest Medicare Advantage menus in the country, and nearly all the marketing a 64-year-old in Peoria receives is for those plans. That is not a complaint about them. It does mean the Supplement window, which no plan has a reason to advertise, goes by quietly.

It matters most at the north end of the city. Households in Vistancia, Trilogy and along the Happy Valley corridor are more likely to split the year between two states, and a Supplement has no network: it pays wherever Original Medicare pays, in Peoria in February and at the lake house in July. Those are also the households most likely to have the retirement income to carry the premium without strain, and to care more about a predictable ceiling on medical costs than about the lowest monthly outlay. Further south, in the 85345 core and Westbrook Village, more people are year-round residents whose doctors are all within a few miles, and a well-chosen Advantage plan is often the better fit.

Premiums are a local question too. Supplement pricing is filed by ZIP area, and plan line-ups along the Peoria, Glendale and Sun City seam change within a mile, which I covered in Do Medicare plans change by ZIP code in Peoria? Price your own ZIP, not a neighbor’s. The Peoria service-area page has the north and south picture, and the Glendale page covers the other side of the seam.

If the choice inside the window comes down to two letters, it is usually G and N. Plan G leaves you the 2026 Part B deductible of $283 and little else. Plan N trades a lower premium for small copays and exposure to excess charges, which are legal in Arizona. Plan G vs. Plan N in Arizona runs that comparison, and Medicare Advantage vs. Medigap is the evergreen version of the bigger fork.

What to do if your window opens in the next six months

1. Find your Part B start date. If you have not signed up, decide when you will, because that date sets everything else. The enrollment timeline tool shows the choices for your birth month.
2. If you or your spouse are still working, settle the Part B question first. Group coverage from a current job at an employer with 20 or more employees usually means waiting on Part B, which keeps the six months intact for the day you retire.
3. Write the last day of the window on the calendar. Six months from the first of the month Part B begins. That date is the one that cannot move.
4. Decide between the two roads while both are open. Compare a Supplement plus a Part D plan against the Advantage plans that include your doctors, using your real prescriptions and your real travel pattern.
5. Ask about the pre-existing condition waiting period, and have proof of your prior coverage ready so it does not apply.
6. Remember Part D has its own clock. A Supplement does not include drug coverage, and the Part D late penalty is separate. Do I need Medicare Part D if I don’t take any prescriptions? explains why.

If you would like to lay your own dates out with someone, that is a short conversation and you do not need to be ready to buy anything. Book a Turning-65 planning call, or call (602) 844-6002. I am independent, the office is in Anthem, roughly 25 minutes from north Peoria out the Carefree Highway and down Lake Pleasant Parkway, and most of these conversations happen by phone. Start it a few months before your birthday month, not in month five of the six.