Updated: Sep 9, 2026
Key Takeaways
- General Enrollment is January 1–March 31, with coverage the next month.
- The current-employment Part B SEP generally ends eight months after employment or coverage ends, whichever is first.
- Medigap has separate enrollment protections and possible underwriting.
Match the period to the action
| Period | Who and when | Action and typical effective date |
|---|---|---|
| Initial Enrollment Period | Usually three months before 65, birthday month, and three months after | Enroll in A/B; Part B starts in birthday month if enrolled beforehand, otherwise following month |
| General Enrollment Period | January 1–March 31 for eligible people who missed their A/B window | Enroll; coverage generally starts the following month; penalties may apply |
| Current-employment Part B Special Enrollment Period | While covered through current employment, or generally up to eight months after work or coverage ends, whichever is first | Enroll in Part B; confirm start date with Social Security |
| Fall Open Enrollment | October 15–December 7 | Join, leave, or change eligible Advantage or Part D coverage; generally effective January 1 |
| Medicare Advantage Open Enrollment | January 1–March 31 for people already in Advantage | One change to another Advantage plan or return to Original Medicare with an opportunity for Part D; generally effective next month |
| Medigap Open Enrollment | Six months beginning the first month you are 65 or older and enrolled in Part B | Buy a Medigap policy with federal open-enrollment protections; separate from annual plan shopping |
First-of-month birthdays affect the initial calendar. Other Special Enrollment Periods exist for specific circumstances; this table is not an exhaustive eligibility determination.
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If you are leaving a job
Ask the employer benefits administrator who pays first, when active-employment coverage ends, and what documentation Social Security needs. The eight-month Part B clock uses employment or qualifying coverage ending, whichever occurs first. COBRA and retiree coverage do not restart or extend it.
Part D has different creditable-drug-coverage rules and deadlines. Do not use the eight-month Part B rule to decide how long you can go without drug coverage. Arrange the transition before current coverage ends to reduce the risk of a gap.
Check Medigap before switching
Returning to Original Medicare does not automatically guarantee you can buy any Medigap policy. Outside protected enrollment or guaranteed-issue situations, underwriting may apply depending on state rules. Trial rights can apply in particular circumstances. Verify your right to buy the desired policy before cancelling coverage.
Complete and confirm enrollment
Use Medicare's enrollment guidance and its links to Social Security's application steps. Keep application confirmations and employer documents. Verify the effective date on your notice or Medicare card rather than assuming submission means immediate coverage.
See eligibility rules and Part B penalty calculations for the related questions. Part A, Part B, and Part D penalties have different rules; a missed deadline does not produce the same charge under every part.
Frequently Asked Questions
Can I use fall Open Enrollment to sign up for Part B for the first time?
Does COBRA extend the eight-month Part B window?
ARTICLE SOURCES
Retire Companion requires writers to use primary sources to support their work. These include white papers, government data, original reporting, and interviews with industry experts. We also reference original research from other reputable publishers where appropriate. You can learn more about the standards we follow in producing accurate, unbiased content in our editorial policy.
- Medicare. When can I sign up for Medicare?
- Medicare. When does Medicare coverage start?
- Medicare. Buying a Medigap policy
- Medicare. Avoid late enrollment penalties
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