Your Initial Enrollment Period (IEP)
Your first and most important window is the Initial Enrollment Period: seven months built around your 65th birthday — the three months before your birthday month, your birthday month itself, and the three months after. During this window you can enroll in Part A (hospital), Part B (medical), Part D (prescription drugs), or a Medicare Advantage plan.
Enrolling in the three months before your birthday month gets coverage in place the month you turn 65, which is the smoothest path. Wait until later in the window and your start date shifts later. If you're already receiving Social Security benefits when you turn 65, you're typically enrolled in Parts A and B automatically — but it's worth confirming rather than assuming.
Still working at 65? The Special Enrollment Period
If you (or your spouse) are still working at 65 and covered by an employer group health plan from an employer with 20 or more employees, you can generally delay Part B without penalty. When that employment or coverage ends, a Special Enrollment Period opens: you have eight months to sign up for Part B penalty-free.
Two traps to sidestep. First, COBRA and retiree coverage don't count as current employer coverage for this purpose — the eight-month clock starts when active employment ends, not when COBRA does. Second, if your employer has fewer than 20 employees, Medicare generally becomes primary at 65, and delaying can leave gaps. When in doubt, ask your benefits administrator to confirm how your plan coordinates with Medicare before you delay anything.
The annual windows: AEP and OEP
The Annual Enrollment Period runs October 15 through December 7 every year. This is the big one for fine-tuning: you can join, switch, or drop Medicare Advantage and Part D drug plans, with changes taking effect January 1. Since plan networks, formularies, and costs change each year, reviewing your plan's Annual Notice of Change every fall is time well spent.
The Medicare Advantage Open Enrollment Period runs January 1 through March 31. If you're already in a Medicare Advantage plan and it isn't working out, you get one switch — to another Advantage plan or back to Original Medicare (with the option to add a Part D plan). There's also a General Enrollment Period in the same January–March window for people who missed their IEP entirely, with coverage beginning the month after you enroll.
Penalties: why the calendar has teeth
The Part B late-enrollment penalty adds 10% to your premium for each full 12-month period you were eligible but not enrolled (and not covered by qualifying employer insurance) — and it usually lasts for as long as you have Part B. The Part D penalty works monthly: roughly 1% of the national base premium for every month you went without drug coverage or other creditable coverage, also generally permanent.
The theme is that these penalties aren't one-time fees — they ride along for life. Which is why the single best Medicare habit is simply knowing which window you're in and what it's for.
The golden window most people miss: Medigap
If you choose Original Medicare, a Medigap (Medicare Supplement) policy helps cover the out-of-pocket costs Parts A and B leave behind. Federal rules give you one guaranteed window to buy: your Medigap Open Enrollment Period, the six months starting the month you're 65 or older and enrolled in Part B. During it, insurers must sell you a policy regardless of your health history.
Outside that window, in most states, Medigap insurers can use medical underwriting — meaning they can charge more or decline coverage based on health. Some states offer extra protections, but the safe assumption is that the six-month window is the one time the door is guaranteed open. If Original Medicare plus Medigap is your plan, don't let that window close unexamined.