https://www.fool.com/amp/retirement/2018/11/24/how-much-do-you-really-know-about-medicare.aspx

Don't miss your first eligibility period, because ...

If you don't enroll during one of the regular or special enrollment periods, you'll often have to pay a penalty when you do sign up. For each year you were eligible for Medicare coverage but didn't enroll, your monthly Part B premium will increase by 10% -- for life.

Although you can't enroll whenever you want to, there are several times when you can sign up. If you're enrolling in Medicare for the first time, you have from three months before your 65th birthday until three months after your 65th birthday to sign up. There also are special enrollment periods that apply in certain situations, such as a change in family status or the loss of employer coverage for those still working when they turn 65. These special enrollment periods give you another opportunity to enroll in Medicare Parts A and B or an Advantage (aka 'Part C') plan.

In addition, for Part D plans in particular, there are other situations that can trigger the special enrollment periods, such as moving to a new area that your current provider doesn't serve, or if your current plan stops serving your location.

You also have the opportunity to enroll during the general enrollment period and open enrollment period. The general enrollment period runs from January 1 through March 31 during which time you can only sign up for Medicare Parts A and B. The open enrollment period is more flexible. It runs from October 15 through December 7, and you can switch from original Medicare to an Advantage plan (or vice versa), sign up for or drop Part D coverage, or change to a new Advantage plan.