Your Initial Enrollment Period is seven months long
It starts three months before the month you turn 65, includes your birthday month, and ends three months after it. If you sign up in the first three months, coverage can start on the first day of your birthday month. Sign up later in the window and it starts later.
If you are already receiving Social Security benefits before you turn 65, you are generally enrolled in Part A and Part B automatically, and your card arrives by mail before your birthday month. If you do not receive benefits yet, you sign up yourself, through Social Security.
Source: Medicare.gov: When does Medicare coverage start?
Still working at 65? Check before you delay Part B
If you or your spouse have health coverage through current employment, you may be able to delay Part B and sign up later without a penalty. That later window is a Special Enrollment Period: any time you are still covered through the job, or within eight months after the job or the coverage ends, whichever comes first.
Two details catch people. COBRA and retiree coverage are not coverage from current employment, so they do not keep that window open. And at an employer with fewer than 20 employees, Medicare usually pays first, so delaying Part B can leave you with far less coverage than you expect. Ask the plan’s benefits administrator how it works with Medicare before you decide.
Source: Medicare.gov: When can I sign up for Medicare?
The six months that matter for Medigap
Your Medigap Open Enrollment Period is six months long and starts the first month you have Part B and are 65 or older. During it, an insurance company cannot refuse to sell you a Medigap policy it offers, and cannot use your health to decide whether to accept you. It happens once. It does not repeat every year.
After it ends, buying a Medigap policy can be harder and cost more. That is why the date you start Part B matters: it starts this clock.
Source: Medicare.gov: When can I buy a Medigap policy?
Drug coverage has its own penalty
If you go 63 days or more in a row without Part D or other creditable drug coverage after your Initial Enrollment Period ends, a late enrollment penalty is added to your Part D premium, generally for as long as you have Part D. Coverage from an employer or union plan may count as creditable; the plan tells you each year whether it does.
Source: Medicare.gov: Avoid late enrollment penalties
Every year after: October 15 to December 7
During Medicare Open Enrollment you can switch between Original Medicare and a Medicare Advantage plan, change Advantage plans, and join, switch or drop a drug plan. Changes take effect on January 1.
Plans change their costs and the drugs they cover from year to year, and they send an Annual Notice of Change in the fall. Reading it is the simplest way to catch a change before it reaches you.
From January 1 to March 31, if you are in a Medicare Advantage plan, you can switch to another Advantage plan or return to Original Medicare, once.
Source: Medicare.gov: Open Enrollment
Medicare Planning at Osprey starts with the same questions this page does: when you turn 65, whether you are still working, and what coverage you have now. Medicare Planning