As your retirement gets closer, it's normal to feel uncertain. The next phase of your life holds a lot of wild cards, and these can be difficult to plan for. Of all the question marks, health care is one of the biggest. The average couple spends $280,000 on healthcare in retirement. It's a line item too expensive to ignore.

Medicare is one way to soften the financial blow. It's an opportunity for guaranteed and affordable health insurance. But only if you follow a strict set of rules.

The different parts of Medicare

Medicare is the government's health insurance program. Most people sign up at 65, but if you have certain disabilities, it may be possible to enroll sooner. You may also qualify with end-stage renal disease. Let's look at the different parts of the program.

Part A - This is often called "hospital insurance." Part A covers hospital stays and short-term care in a skilled nursing facility. It also includes some services at home. Learn more about the specifics here.

Part B - This is also called "medical insurance." Part B covers visits with certain doctors, outpatient care, and medical supplies. Preventative services are also included. It's important to know you're responsible for 20% of these expenses. The coverage details are here.

Part D - These are Medicare-approved private plans covering prescription drugs.

TIP: Parts A and B don't cover everything. You'll still have deductibles, coinsurance and copayments, and are still on the hook for:

• Long-term care
• Most dental care
• Eye exams
• Dentures
• Cosmetic surgery
• Acupuncture
• Hearing aids and exams for fitting them
• Routine foot care

Mistake #1 - Failing to sign up when you're first eligible

If you haven't reached your full retirement age yet, enrolling in Medicare may not be top of mind. The problem is, it's easy to miss a key deadline. You have a seven-month window to sign up for Part A and Part B. This includes:

• Three months before you turn 65
• The month you turn 65
• Three months after you turn 65

If you don't enroll in Medicare Part B when you're first eligible, you may face a late enrollment penalty. It could be 10% more monthly premiums for each 12-month you were eligible but didn't sign up. This penalty stays with you for as long as you're using Medicare Part B.

It's possible you'll also have to wait until January 1 - March 31 to sign up. This means your coverage wouldn't start until the following July.

If you didn't sign up because you can't afford it, your state may offer resources to help cover deductibles, coinsurance, and copayments. You can learn more about the four types of Medicare Savings Programs here.

Mistake #2 - Missing the Medigap enrollment window

If you're healthy now, it may be hard to imagine a different scenario. But the truth is, no one has a crystal ball. Things may change faster than you expect. In an ideal world, Medicare would cover all your healthcare expenses. Unfortunately, the realities of coverage look a lot different.

Medigap is a private insurance policy to cover copayments, coinsurance, or deductibles. For example, Medigap may cover the 20% Part B deductible you'd normally have to pay. Like other types of insurance, you'll qualify for the best plan when you're younger and healthier.

Tip: There's a six-month Medigap enrollment window. The clock starts ticking once you've enrolled in Medicare Part B. After that, it's possible you won't qualify for a plan. Even if you do, it may be more expensive.

Mark your calendar for these critical deadlines

Missing the deadline to sign up for Medicare Part A, Part B, or Medigap is more than a simple mistake. It will cost you a lot more money for years to come. Do yourself a favor by setting more than one reminder. You'll sleep better knowing you'll be ready when the time comes.

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