Medicare runs on a fixed fall calendar, and the next few months follow it closely. Sometime in September, the Centers for Medicare & Medicaid Services will publish the finalized 2027 Medicare Advantage and Part D plan lineup, along with the final average premiums that replace the preliminary figures insurers have been working from all summer. Six weeks after that data lands, on October 15, the annual window opens for comparing and switching Medicare coverage, and it closes December 7. For anyone on Medicare, or helping a parent through the process, that stretch between the September release and early December is the entire comparison window for the year.
CMS’s September Numbers Replace July’s Preview
On July 28, 2026, CMS released preliminary Part D bid information for 2027. That release is aimed at insurers, not shoppers: it is the industry-facing data plan sponsors use to finalize what they will actually offer and charge, not a number any household should expect to see on a bill. It arrives roughly two months ahead of the plan-by-plan detail a shopper actually needs, and it is not final.
CMS’s own fact sheet on the 2027 bid cycle states plainly that the agency has not yet released final 2027 premiums or plan offerings, and that those details become available in September, when CMS publishes the Medicare Advantage and Part D landscape. Until that landscape posts, any premium figure circulating publicly is a preview built on bid math, not the plan-by-plan number a household will actually see for their own ZIP code and plan.
The gap between the two releases matters because bid data and landscape data answer different questions. Bid data tells insurers and analysts where costs are trending nationally, months before any single plan finalizes its own premium, deductible, or drug list. Landscape data tells an individual household what is actually offered where they live: which carriers are still in their county, what each plan’s premium and coverage will be, and whether a plan they currently use is even being renewed for 2027. A household that tries to shop off the July bid numbers is comparing an industry forecast to a decision that has not been finalized yet.
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The Letter That Starts the Countdown
Once the landscape data is close to final, current Medicare plans mail their members a Plan Annual Notice of Change, commonly called the ANOC. Medicare.gov’s own guidance on the notice confirms it arrives each fall in September and spells out any changes to coverage, cost, or service area that take effect the following January. It is the first document that tells an existing Medicare Advantage or Part D member, in writing, how their own plan is changing for 2027, rather than what the market as a whole is doing.
The ANOC is meant to be read against the year now ending: same premium or higher, same drug list or a shorter one, same doctors in network or not. Medicare’s own guidance is specific about the job of that letter: review any changes to decide whether the plan will still meet your needs in the next year, and if the notice does not arrive, contact the plan directly rather than assume nothing has changed. A household that reviews it and finds their current plan still fits does not have to do anything else. A household that finds a gap has roughly two weeks of runway before the comparison window opens, which is why insurers and CMS both treat the September mailing as the practical start of enrollment season, even though the official period begins later.
October 15 to December 7: The Window That Counts
The comparison window itself does not move with the calendar of any single year’s data release. CMS’s Medicare Open Enrollment page states it without qualification: every year, Medicare’s open enrollment period is October 15 through December 7, and that stretch is when anyone with Medicare can join, switch, or drop a Medicare Advantage or Part D drug plan for the following year. The same CMS page notes that information on next year’s plans becomes available beginning in October, which is when the September landscape data actually becomes usable for side-by-side shopping rather than just industry analysis.
Households can search and compare specific 2027 plans, once they post, through Medicare’s own Plan Compare tool. Any change made inside the October 15 to December 7 window takes effect January 1, 2027, and any change missed carries the current plan forward into the new year by default. CMS does not build in a second general window for this decision — the six weeks bounded by the September data release on one side and December 7 on the other is the one the agency itself designates for it.
Not every household needs to act inside that window. CMS’s own enrollment guidance is direct on this point: if a current plan is satisfied with its ANOC, still being offered, and still meets a member’s needs for next year, there is nothing further to do. The window exists for the household whose plan is changing, disappearing, or no longer the best fit once the real September numbers replace the July preview — and for that household, the six weeks between the landscape release and December 7 is the only chance to act before the new plan year begins.
This article was produced with AI assistance and reviewed by a human editor. Figures are linked to their primary sources; where a claim could not be verified from the public record, we say so.
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