The right enrollment window can prevent a costly gap.

Your first window: seven months around age 65

The Initial Enrollment Period generally begins three months before the month you turn 65, includes your birthday month, and ends three months afterward. If your birthday is the first day of a month, Medicare treats your eligibility month as the preceding month, shifting the window earlier.

For example, an August 15 birthday generally gives a May 1–November 30 enrollment window. Enrolling in Part B before August generally allows an August 1 start. Enrolling during your birthday month or one of the following three months generally starts Part B the next month. Different rules apply to some disability and kidney-disease situations.

Some people receiving Social Security or Railroad Retirement benefits are enrolled automatically; others must apply. Read the notice and confirm both Parts A and B and their effective dates. Do not assume a Medicare card means every coverage decision is complete.

Still working? Check before delaying Part B.

Coverage based on your or your spouse’s current employment may allow a Special Enrollment Period. Generally you can enroll while that coverage continues or during the eight months after the employment or coverage ends, whichever happens first. Coverage coordination depends on circumstances such as employer size. Ask the benefits administrator and Social Security before delaying.

COBRA, retiree coverage, Marketplace coverage, and VA coverage are not the same as current-employment coverage for this Part B enrollment rule. Choosing COBRA does not restart the eight-month clock. Part D’s creditable-drug-coverage deadline is separate and can be much shorter.

If you contribute to an HSA, seek guidance before Medicare enrollment: Medicare coverage, including retroactive premium-free Part A, can affect eligibility to contribute.

Annual Election Period: October 15–December 7

This fall window is also called Medicare Open Enrollment. During October 15–December 7, 2026, eligible people can select Medicare Advantage or Part D coverage for January 1, 2027. You can change plans, move from Original Medicare to Medicare Advantage, or return to Original Medicare. Requests must reach the plan by December 7.

Review the Annual Notice of Change even if you like your current plan. Check next year’s doctors, hospitals, medications, pharmacies and total costs. AEP is not an automatic right to buy Medigap without underwriting. Before leaving Medicare Advantage, check your Medigap rights and separate drug coverage.

Medicare.gov: fall Open Enrollment ↗

January–March is a different window

Medicare Advantage Open Enrollment runs January 1–March 31 for people already in Medicare Advantage. Generally, one change is allowed: another MA plan or Original Medicare with an opportunity to join Part D. Coverage generally changes the first of the next month. People with Original Medicare cannot use this window to join MA. New MA enrollees may also have an opportunity during their first three months with Parts A and B.

This is distinct from the General Enrollment Period for Part A/B, even though the dates overlap.

Enrollment periods at a glance

What happens if you enroll late?

• Part B: generally an extra 10% of the standard premium for each full 12-month period you could have had Part B but did not enroll. You generally pay it for as long as you have Part B. Qualifying SEPs and other exceptions can avoid the penalty.
• Part A you must buy: the monthly premium may increase 10%, usually for twice the number of years enrollment was delayed. This is different from premium-free Part A.
• Part D: after your initial enrollment window, going 63 consecutive days or more without Part D or other creditable prescription coverage can trigger a penalty. It is generally 1% of the national base beneficiary premium for each full uncovered month, added to your premium, usually for as long as you have Part D. Extra Help and other exceptions can apply.

A coverage gap can be costly even before a penalty starts. Keep employer creditable-coverage notices and confirm effective dates before ending existing coverage.

Special Enrollment Periods: the event determines the clock

SEPs are not a general permission to change any plan whenever you want. Examples:

• Move outside your service area: generally through two full months after the move; telling the plan beforehand can open the window the preceding month.
• Leave employer or union coverage: generally two full months after the month coverage ends for joining MA or Part D.
• Lose Medicaid: generally three full months after loss or notification, whichever is later.
• Plan nonrenewal: generally December 8 through the last day of February.
• Medicaid or Extra Help: certain monthly Part D changes; eligible full-benefit dual enrollees may have an integrated D-SNP opportunity. Not unrestricted monthly MA switching.

The eight-month current-employment Part B SEP is a different rule. Confirm the event, allowed change and deadline; keep notices. Other events can qualify.

When life changes, ask about an SEP promptly.

A move, loss of employer coverage or Medicaid, plan termination, or certain exceptional circumstances may create enrollment rights. The Part A/B SEP after losing Medicaid differs from a private-plan SEP. Full Medicaid or Extra Help may provide certain monthly drug-plan opportunities; this does not mean unrestricted monthly switching among all Medicare Advantage plans.

Compare HMO, PPO, PFFS, SNP and MSA plan structures →

Read about Medicaid, LIS, and related SEPs →

Sources: Medicare & You 2026 and 2027, sections 1, 4, 6 and 7. Enrollment-period details checked against Medicare.gov October 17, 2026.

Official Medicare enrollment guidance ↗ · Check an SEP ↗