Medicare Enrollment Periods and Deadlines Explained
Every Medicare enrollment window in one place: the Initial Enrollment Period, Annual Enrollment October 15 to December 7, Medicare Advantage Open Enrollment, Special Enrollment Periods, the Medigap window and the edge cases.
Medicare only lets you join or change coverage at set times. The big ones are your seven-month Initial Enrollment Period around your 65th birthday, the Annual Enrollment Period from October 15 to December 7 each year (changes start January 1), the Medicare Advantage Open Enrollment Period from January 1 to March 31, and Special Enrollment Periods triggered by events like retiring or moving. Medicare Supplements follow their own clock: a one-time six-month window that starts when you are 65 and enrolled in Part B. People under 65 on disability, people born on the first of a month and people who must buy Part A have their own rules, covered below. Right now, October 15 to December 7, 2026 is the window to choose coverage for 2027.
Every window at a glance
| Window | When | What you can do |
|---|---|---|
| Initial Enrollment Period | 3 months before your 65th birthday month, that month, and 3 months after | Sign up for Parts A and B; join a Medicare Advantage or Part D plan |
| Annual Enrollment Period | October 15 to December 7 each year | Join, switch or drop Medicare Advantage and Part D plans; changes start January 1 |
| Medicare Advantage Open Enrollment | January 1 to March 31 | If you are in a Medicare Advantage plan: switch to another one, or return to Original Medicare and add Part D (one change) |
| General Enrollment Period | January 1 to March 31 | Sign up for Part B (or Part A) if you missed your first window; coverage starts the month after you sign up |
| Special Enrollment Periods | Usually 2 to 8 months after a qualifying event | Enroll or change plans after events like losing employer coverage or moving |
| Medigap Open Enrollment | 6 months starting the month you are 65 and enrolled in Part B | Buy any Medicare Supplement sold in Colorado without health questions |
Your Initial Enrollment Period
The Initial Enrollment Period is seven months long. If you turn 65 on May 5, it runs from February 1 through August 31. Sign up during the three months before your birthday month and Medicare starts on the first day of your birthday month. Sign up during your birthday month or the three months after, and coverage starts the first of the month after you sign up.
If your birthday is on the first of the month, Medicare treats you as turning 65 in the month before. Someone born June 1 has an Initial Enrollment Period of February 1 through August 31, and coverage can start May 1. Our Medicare enrollment date calculator on the Tools page works out your exact window from your birth date.
If you are already receiving Social Security, you are enrolled in Parts A and B automatically. You still need to choose a Medicare Advantage plan, or a Medicare Supplement and Part D plan, yourself.
Your first chance to join a Medicare Advantage plan, which Medicare calls the Initial Coverage Election Period, usually matches your Initial Enrollment Period. If you delay Part B because you are still working, it is the three months just before your Part B starts.
If you are under 65 and on disability
If you get Social Security or Railroad Retirement disability benefits, you are enrolled in Parts A and B automatically after 24 months of benefits, and your card arrives about three months before. People with ALS get Medicare the month their disability benefits start. People with End-Stage Renal Disease have separate rules tied to dialysis or a transplant, and must sign up themselves.
Your disability Initial Enrollment Period is the seven months around your 25th month of disability benefits, and you can join a Medicare Advantage or Part D plan then. When you turn 65 you get a fresh Initial Enrollment Period, and a fresh chance to choose plans.
Colorado is one of the states that requires Medigap companies to sell to people under 65 on Medicare because of a disability, according to the Colorado Division of Insurance. Rates can be higher than at 65. At 65 you get a new six-month Medigap Open Enrollment Period, so it pays to shop again then.
If you have to buy Part A
Most people get Part A premium-free after 40 quarters of work. If you or your spouse have fewer, you can buy it: in 2026 the premium is $311 a month with 30 to 39 quarters and $565 with fewer than 30 (CMS). If you buy Part A, you must also have Part B.
Buy it when you are first eligible. If you wait, Part A has its own penalty: 10 percent more on the premium, paid for twice the number of years you could have had it and did not. Health First Colorado pays the Part A premium for people who qualify for QMB, and QDWI pays it for certain working people with disabilities.
The Annual Enrollment Period, October 15 to December 7
This is the window everyone on Medicare shares. You can join a Medicare Advantage plan, switch from one to another, drop Medicare Advantage and return to Original Medicare, or join, switch or drop a Part D plan. Whatever you choose takes effect January 1.
Plans mail an Annual Notice of Change by September 30 listing what is different next year: premiums, copays, networks and drug lists. Plans renew automatically, so if you do nothing you keep the same plan with its new terms. That is why I review every client's coverage each fall rather than assuming last year's plan still fits.
Medicare Advantage Open Enrollment, January 1 to March 31
If you are in a Medicare Advantage plan on January 1, you get one more change before March 31. You can switch to a different Medicare Advantage plan, or leave Medicare Advantage for Original Medicare and join a standalone Part D plan. You cannot use this window to move from Original Medicare into Medicare Advantage, and you get one change only. If you are new to Medicare and joined a Medicare Advantage plan, you also get a one-change window during your first three months of Medicare.
Special Enrollment Periods
Life events open their own windows. These are the ones I see most often:
- Losing employer or union coverage, from your own job or your spouse's: 8 months to sign up for Part B without a penalty, and 2 months to join a Medicare Advantage or Part D plan.
- Moving out of your plan's service area, or moving to Colorado: a window to choose a plan that serves your new county.
- Moving into or out of a nursing home or other institution.
- Qualifying for, or losing, Health First Colorado (Colorado Medicaid) or Extra Help.
- Your plan leaving Medicare or your area, or Medicare ending its contract with the plan.
- Joining a plan with a 5-star overall rating, once a year.
A worked example: retiring from a Colorado employer
Picture someone in Brighton who is 66, covered by a group plan through their own job at a company with more than 20 employees, and retiring with coverage ending June 30, 2027. Here is the order that avoids gaps and penalties. This is an illustration; check your own dates.
- April or May 2027: sign up for Part B with Social Security, using the employer form that confirms group coverage from current work, and ask for a July 1 start date. Part A may already be in place.
- Before June 30: choose a path. A Medicare Advantage or Part D plan can be chosen now for a July 1 start. If you want a Medicare Supplement, ask each company how early it accepts an application so the policy can start July 1.
- July 1 to December 31, 2027: the six-month Medigap Open Enrollment Period, because it starts with Part B. Any Medigap plan sold in Colorado must accept you at its standard rate.
- By August 31, 2027: the last day to join a Medicare Advantage or Part D plan under the 2-month window, if not done already. Waiting longer without creditable drug coverage starts the Part D penalty clock.
- February 28, 2028: the end of the 8-month Part B window. Signing up after that means waiting for the General Enrollment Period and a possible penalty.
The General Enrollment Period
If you missed your Initial Enrollment Period and do not qualify for a Special Enrollment Period, you can sign up for Part B between January 1 and March 31. Coverage starts the first of the month after you sign up, and the late penalty may apply. In 2026 that penalty is 10 percent of the $202.90 standard premium for each full 12 months you went without, so two years late adds $40.58 a month for as long as you have Part B.
The Medigap window, and why it matters most here
Your Medigap Open Enrollment Period is six months long and starts the first month you are both 65 and enrolled in Part B. During it you can buy any Medicare Supplement sold in Colorado, at the same price as a healthy person. It happens once and does not repeat.
Colorado has no birthday rule or yearly window to switch Medigap policies without health questions (NAIC chart as of May 11, 2026). Colorado does add one guaranteed issue right of its own: if you had Medicaid when you first became eligible for Medicare and later lose it, the Colorado Division of Insurance says you can buy a Medigap policy without underwriting. Federal trial rights also apply: if you joined a Medicare Advantage plan when first eligible at 65, you can switch to a supplement without health questions within your first year.
Which deadline applies to you?
- Turning 65 and not working: your Initial Enrollment Period, then your Medigap window.
- Working past 65 with group coverage from current work: a Special Enrollment Period when the job or coverage ends, and your Medigap window starts with Part B.
- Already on Medicare and unhappy with your plan: October 15 to December 7, or January 1 to March 31 if you are in Medicare Advantage.
- Moving, losing Medicaid or qualifying for Extra Help: a Special Enrollment Period tied to that event.
- Missed everything: the General Enrollment Period, January 1 to March 31.
The bottom line
Most people only need three dates: their Initial Enrollment Period, the October 15 to December 7 Annual Enrollment Period every year, and their one-time Medigap window. For 2027 coverage, the window is open now through December 7, 2026. If you are retiring, on disability, born on the first of the month or buying Part A, the dates shift, so check them early. If a life event has you wondering which window applies, call us and we will check before a deadline passes.
Want to go through this with someone?
Lisa Asmussen at our Brighton office will go through your doctors, your prescriptions and your real numbers, and explain plainly what we found. Take the Stress Out of Medicare
Prefer to learn in a group? See our Medicare education sessions, in person in Brighton or online, and official Medicare forms and publications on our Resources page.
Sources
- When can I sign up for Medicare?, Medicare.gov (CMS)
- Avoid late enrollment penalties, Medicare.gov (CMS)
- 2026 Medicare Parts A and B premiums and deductibles, Centers for Medicare & Medicaid Services
- State Medigap rules: birthday rule and coverage under 65 (as of May 11, 2026), National Association of Insurance Commissioners
- 2025-2026 Colorado Medigap Policies (Medicare Supplement guide), Colorado Division of Insurance
- 2026-2027 Colorado Medigap Policies (Medicare Supplement guide), Colorado Division of Insurance
Please note: We do not offer every plan available in your area. Currently we represent 10 organizations which offer 106 products in your area. Please contact Medicare.gov, 1-800-MEDICARE, or your local State Health Insurance Program (SHIP) to get information on all of your options. Headwaters Health and Wealth is not connected with or endorsed by the United States government or the federal Medicare program. Medicare has neither endorsed nor reviewed this information.