Medicare's enrollment periods, defined with sources. Initial, General, Special, and the two different windows both called Open Enrollment, plus a short path to the one that is actually yours.

Part eight of Healthcare in Retirement, and this one is a glossary I have wanted to write for a year. It exists because of a phone call that came in last September. The caller had one question: "What is AEP?" And another, in February: "I wasn't sure if I missed the deadline or not." Both of them had been told to watch for open enrollment. Both of them were watching the wrong one.

The word that means three things

Here is the problem in one paragraph. Medicare's own website calls the fall window, October 15 to December 7, "Open Enrollment." The federal regulation calls that same window the "annual coordinated election period," which is why everyone in the industry says AEP. Medicare then uses "Medicare Advantage Open Enrollment" for a completely different window in January. And the Health Insurance Marketplace, the one for people under 65, runs its own "Open Enrollment" from November 1 to January 15. Your employer's open enrollment is set by your employer and has nothing to do with any of them.

Same word, four calendars. So let me define every period Medicare actually has, with the dates, who it is for, and when coverage starts, each one linked to the source.

The periods, defined

Initial Enrollment Period. Medicare: "It lasts for 7 months, starting 3 months before you turn 65, and ending 3 months after the month you turn 65." Sign up in the three months before your birthday month and coverage begins the month you turn 65. Sign up in your birthday month or the three after, and coverage begins the first of the following month. Part A, if you qualify for it with no premium, starts the month you turn 65, or the month before if your birthday is on the first. The Advantage rules have their own name for the same window, the "initial coverage election period," which you will only ever see in the fine print.

General Enrollment Period. This is the one for people who missed the first one and have none of the situations the rules recognize. Medicare: "You can sign up between January 1-March 31 each year. This is called the General Enrollment Period. Your coverage starts the month after you sign up." Notice what it is not. It is not the fall window, and it is not a plan-shopping window. It is the door for Part A and Part B themselves, and it usually comes with a penalty, which we will get to.

Open Enrollment, the fall one. October 15 to December 7, and "The changes you make during Open Enrollment are effective January 1 of next year." In the regulation this is the annual coordinated election period, and the rule spells out what you may do in it: change "from an MA plan to Original Medicare or to a different MA plan, or from Original Medicare to an MA plan," and pick up a drug plan if you go to Original Medicare. Any direction, as many times as you like before December 7. The last choice you make is the one that counts.

Medicare Advantage Open Enrollment Period. The January one. Medicare's calendar: "Between January 1 – March 31, people in a Medicare Advantage Plan can switch to a different Medicare Advantage Plan (with or without drug coverage), or to Original Medicare." Three limits. You must already be in an Advantage plan. You get one change, not several. And you cannot use it to go from Original Medicare into an Advantage plan. It is a do-over for people who picked an Advantage plan in the fall and regret it, and only for them.

Medigap Open Enrollment Period. Not a Medicare period at all, strictly, but it shares the word. "This is the 6-month period that starts the first day of the month you're 65 or older and signed up for Part B." One time. We covered what happens after it closes in part five.

Special Enrollment Periods. These open when something happens to you. The ones that come up most:

• You worked past 65 with employer coverage. "Once you stop working (or lose your health insurance, if that happens first) you have an 8-month Special Enrollment Period (SEP) when you can sign up for Medicare (or add Part B to existing Part A coverage)." The same page adds that the clock starts "even if you choose COBRA," which is the whole point of part one.
• You left employer or union coverage and need a plan, not just Part B. "Your chance to join lasts for 2 full months after the month your coverage ends." Eight months for Part B, two months for the plan. People confuse those constantly.
• You moved out of your plan's service area. "Your chance to switch begins when you move and continues for 2 full months after you move." Tell the plan before you move and it starts a month earlier.
• Your plan left Medicare. "Between December 8 and the last day in February of the following year." Monday's article, if that letter is yours.
• A five-star plan is available where you live. "You can use this Special Enrollment Period only once between December 8 and November 30 the following year."

What it costs to miss one

I am not an advisor. I just did the multiplication. Medicare's penalty page says "You'll pay an extra 10% for each year you could have signed up for Part B, but didn't." The statute counts full 12-month periods, and the penalty lasts as long as you have Part B. On the 2026 standard premium of $202.90, two full years late is 20 percent, which at 2026 rates is $40.58 a month. It recalculates every year as the standard premium rises, and it lasts as long as you have Part B. If you have to buy Part A, the same page says the premium "may go up 10%" and "You'll have to pay the penalty for twice the number of years you didn't sign up." And the drug penalty is "an extra 1% for each month (that's 12% a year)", which we worked out in full in part four.

The caller who wanted to know "why I have a late enrollment penalty when I had creditable coverage from my previous employer group plan" had a real case. Creditable coverage is exactly what erases that penalty. She needed the paperwork proving it, and she needed someone to file it. That is a fixable problem. Missing the General Enrollment Period twice is not, because each year you wait adds another 10 percent, permanently.

Which one is yours

• You are turning 65 and not working. Initial Enrollment Period. Seven months. Sign up early in it.
• You are 65 or past it and still on a group plan. Probably no action now, and an 8-month Special Enrollment Period later. Part two of this series has the arithmetic on whether to wait.
• You are on Medicare and your plan's letter says costs are changing. The fall window, October 15 to December 7.
• You picked an Advantage plan in the fall and it is not working. January 1 to March 31, one change, Advantage members only.
• You missed Part B entirely and nothing on the Special list applies. General Enrollment Period, January 1 to March 31, coverage the month after, penalty likely.
• Something happened: you moved, your plan left, your job ended. A Special Enrollment Period, and the clock started on the day it happened, not the day you noticed.

What a Certified Medicare Planner® does differently

They never say "open enrollment" without saying which one. When you call in January wanting to change plans, they ask two questions before anything else: are you in an Advantage plan today, and did anything happen to you since October. The first answer decides whether the January window exists for you. The second decides whether a Special period does. They know the 8-month and 2-month clocks run separately and put both on the calendar. And when a penalty shows up that should not, they go find the creditable-coverage letter instead of telling you to live with it. The advisors are at 602-281-3898.

Friday, October 2. Foothills Library, Glendale, 10:30 AM. Mustang Library, Scottsdale, 2:30 PM. No cost. Register at 123easymedicare.com/medicare-workshop or call (877) 220-1089.

Next in Healthcare in Retirement: same pill, same plan, different counter. Why one prescription costs more at the pharmacy across the street on the very same drug plan, what "preferred" actually means, and the one thing your pharmacist is allowed to tell you that the plan cannot stop.

This event is presented by 123EasyMedicare, a brand of American Retirement Advisors, an independent, private organization. It is not sponsored by, endorsed by, or affiliated with Medicare, the Centers for Medicare & Medicaid Services, the Social Security Administration, or any government agency. The libraries are not sponsors of, and are not affiliated with, this event. Educational only; not tax, legal, or insurance advice.

Disclaimer: This article is for educational purposes only. It is not sponsored, endorsed, or otherwise representative of Medicare or the federal Medicare program. American Retirement Advisors is not a government agency. For official Medicare information, visit medicare.gov or call 1-800-MEDICARE (1-800-633-4227).