---
title: "Medicare Enrollment Periods: IEP, AEP, OEP and SEP | ESI"
description: "Every Medicare enrollment window on one page: the 7-month Initial Enrollment Period, General Enrollment, the fall Annual Enrollment Period, Medicare…"
url: https://eseniorinsurance-demo-v2.pages.dev/medicare-enrollment-periods
dateModified: 2026-10-06
---

# Medicare Enrollment Periods: IEP, AEP, OEP and SEP

**There are five windows that matter.** Your Initial Enrollment Period is the seven months around your 65th birthday. The Annual Enrollment Period, October 15 to December 7, is when anyone can change Medicare Advantage or Part D plans for the next year. Medicare Advantage Open Enrollment runs January 1 to March 31. Medigap has its own six-month open enrollment when Part B starts. Special Enrollment Periods cover life events such as leaving an employer plan or moving.

| Window | When | What you can do |
|---|---|---|
| Initial Enrollment Period (IEP) | 3 months before your 65th birthday month, your birthday month, 3 months after | Enroll in Parts A and B, a Medicare Advantage plan or a Part D plan for the first time |
| Medigap open enrollment | 6 months starting the month your Part B begins | Buy any Medigap plan sold in your state with no health questions |
| Annual Enrollment Period (AEP) | October 15 to December 7 every year | Join, switch or drop a Medicare Advantage or Part D plan for January 1 |
| Medicare Advantage Open Enrollment (OEP) | January 1 to March 31 | If you are in a Medicare Advantage plan: switch to another one, or return to Original Medicare and add a Part D plan (once) |
| General Enrollment Period (GEP) | January 1 to March 31 | Enroll in Part B if you missed your IEP; coverage starts the month after you sign up, and a late penalty may apply |
| Special Enrollment Periods (SEP) | Triggered by an event | Enroll or change plans after losing employer coverage (8 months), moving out of your plan’s area, a plan leaving the market, qualifying for Extra Help, and other events |

## When coverage starts

Enroll in the three months before your birthday month and Medicare starts on the first day of your birthday month (the first of the month before if your birthday is on the 1st). Enroll during your birthday month or the three months after and coverage starts the first of the month after you sign up. Use the [enrollment-window finder](https://eseniorinsurance-demo-v2.pages.dev/tools#enrollment-window) for your exact dates.

## The most common mistakes

- Treating COBRA or retiree coverage as a reason to delay Part B. Only active employment counts, so the Part B penalty clock keeps running.
- Missing the Medigap window. After the six months, carriers can ask health questions and decline you in most states.
- Waiting for AEP to fix a plan you dislike when a Special Enrollment Period already applies.
- Forgetting that Part D has its own penalty after 63 days without creditable drug coverage.

## What we do during AEP

Every fall we review the Annual Notice of Change for each client’s plan, run their medications against the next year’s formularies and show them whether their current plan still wins. Most clients keep their plan; the ones who switch save real money. Either way, the review is free.

Source: [Medicare.gov, joining a health or drug plan](https://www.medicare.gov/basics/get-started-with-medicare/get-more-coverage/joining-a-plan).

## Medicare 101: the complete guide

Every topic a person new to Medicare needs, each on its own page, written and reviewed by Christian Brindle.

- [Medicare 101: Parts A and B](https://eseniorinsurance-demo-v2.pages.dev/medicare-101)
- [Who is eligible for Medicare](https://eseniorinsurance-demo-v2.pages.dev/medicare-eligibility)
- [How to enroll, step by step](https://eseniorinsurance-demo-v2.pages.dev/how-to-enroll-in-medicare)
- [Turning 65 checklist](https://eseniorinsurance-demo-v2.pages.dev/turning-65-medicare-checklist)
- [Medicare vs Medicaid](https://eseniorinsurance-demo-v2.pages.dev/medicare-vs-medicaid)
- [2026 costs and IRMAA](https://eseniorinsurance-demo-v2.pages.dev/medicare-costs-2026)
- [Late-enrollment penalties](https://eseniorinsurance-demo-v2.pages.dev/medicare-late-enrollment-penalties)
- [Part D in 2026: the $2,100 cap](https://eseniorinsurance-demo-v2.pages.dev/medicare-part-d-2026-changes)
- [Medicare Advantage](https://eseniorinsurance-demo-v2.pages.dev/medicare-advantage)
- [Medigap](https://eseniorinsurance-demo-v2.pages.dev/medigap)
- [Part D](https://eseniorinsurance-demo-v2.pages.dev/part-d)
- [Medicare calculators](https://eseniorinsurance-demo-v2.pages.dev/tools)

## Questions people ask about Medicare Enrollment Periods: IEP, AEP, OEP and SEP

**When is the Medicare Annual Enrollment Period?**

**Can I switch Medicare Advantage plans after January 1?**

**What happens if I miss my Initial Enrollment Period?**

**How long is the Medigap open enrollment period?**

**Is there a Special Enrollment Period when I retire?**

Last updated: October 6, 2026 · Reviewed by [Christian Brindle, CEO and Founder](https://eseniorinsurance-demo-v2.pages.dev/about-us#christian-brindle). Published by the Everything Senior Insurance Data Desk. Sources: [Joining a health or drug plan, Medicare.gov](https://www.medicare.gov/basics/get-started-with-medicare/get-more-coverage/joining-a-plan) · [When does Medicare coverage start, Medicare.gov](https://www.medicare.gov/basics/get-started-with-medicare/sign-up/when-does-medicare-coverage-start).