The Medicare Enrollment Timeline: When to Sign Up

Initial Enrollment Period (IEP)

Your Initial Enrollment Period is the seven-month window surrounding your 65th birthday: it starts three months before the month you turn 65, includes your birthday month, and extends three months after. This is your primary opportunity to enroll in Medicare Parts A and B without penalties.

When your coverage begins within this window depends on when you sign up. Enrolling during the three months before your birthday month generally gives you the earliest possible start date. Waiting until after your birthday month may delay the start of coverage.

Special Enrollment Period (SEP)

If you are still working and have health coverage through your employer (or your spouse's employer) when you turn 65, you may not need to enroll in Part B immediately. When that employer coverage ends, you get a Special Enrollment Period — typically eight months from the date the coverage ends or the employment ends (whichever comes first).

COBRA coverage and retiree health plans generally do not qualify for a Special Enrollment Period. This distinction catches many retirees off guard. If you are relying on COBRA after leaving an employer, you may still need to enroll in Part B during your Initial Enrollment Period to avoid penalties.

Medicare Advantage and Part D Enrollment

The Annual Election Period (also called Open Enrollment) runs from October 15 through December 7 each year. During this window, you can join, switch, or drop a Medicare Advantage plan, and join or switch a Part D prescription drug plan. Changes made during this period take effect January 1 of the following year.

There is also a Medicare Advantage Open Enrollment Period from January 1 through March 31. During this window, if you are already in a Medicare Advantage plan, you can switch to a different Medicare Advantage plan or drop your plan and return to Original Medicare (and enroll in a Part D plan). This period exists specifically for people already in Medicare Advantage — it is not for first-time enrollment.

Medigap Open Enrollment Period

This is one of the most consequential — and most commonly misunderstood — enrollment windows in all of Medicare. Your Medigap Open Enrollment Period begins on the first day of the month you are both 65 or older and enrolled in Medicare Part B. It lasts six months.

During this one-time window, insurance companies that sell Medigap policies in your state must sell you a policy regardless of your health status. They cannot charge you more because of pre-existing conditions. They cannot deny you coverage.

After this window closes, medical underwriting may apply in most states. This means an insurer can review your health history, charge higher premiums, or deny coverage entirely based on pre-existing conditions. This is why the Medigap decision is time-sensitive in a way that few other Medicare decisions are.

General Enrollment Period

If you missed your Initial Enrollment Period and do not qualify for a Special Enrollment Period, the General Enrollment Period runs from January 1 through March 31 each year. Coverage begins July 1. Late enrollment penalties may apply, and they are typically permanent — added to your premium for as long as you have Medicare.

The Timeline Matters

Medicare enrollment is not a decision you can make at any time. Each window has specific rules, and the consequences of missing or misunderstanding them can last for years — or permanently. The most common mistake is assuming you can "figure it out later." In Medicare, later often means more expensive.

"The six-month Medigap Open Enrollment Period is the single most important window in Medicare planning. Once it closes, your options may narrow significantly."