Medicare only offers three different types of enrollment periods for Part A & Part B: initial enrollment, special enrollment, and general enrollment periods. If you did not sign up at 65 or when an employer group health plan ended, you will sign up during a general enrollment period.
There is an annual general enrollment period. It runs from January through March each year. If you sign up during this period, your coverage begins July 1. Additionally, you may have a premium increase if you did not have coverage for a full year (this does not include having an employer group health plan ) before signing up. Remember COBRA and retiree plans don’t count as coverage to avoid a premium increase.
The other point you need to remember is about Medicare and Health Savings Accounts. Here is what Medicare says about that situation in their informative resource, Medicare and You 2017:
You can’t contribute to your
HSA once your Medicare coverage begins. If you contribute to your
HSA after your Medicare coverage starts, you may have to pay a
tax penalty. If you’d like to continue contributing to your HSA, you
shouldn’t apply for Medicare, Social Security, or Railroad Retirement
Board (RRB) benefits.
Remember, premium-free Part A coverage begins 6 months back
from the date you apply for Medicare (or Social Security/RRB
benefits), but no earlier than the first month you were eligible for
Medicare. To avoid a tax penalty, you should stop contributing to your HSA at least 6 months before you apply for Medicare. https://www.medicare.gov/pubs/pdf/10050-Medicare-and-You.pdf
Your health coverage in retirement is just as important as your health coverage while you work. You need to read and understand the rules before age 65, so you can decide what action you should take and when you should take it. (If you began receiving Social Security retirement benefits before you turned age 65, you will not need to take any action. Social Security will automatically enroll you in Part A and Part B and send you a red, white and blue Medicare card in the mail. Everyone else will have to contact Social Security to enroll in Medicare.)
You should read Medicare’s Medicare and You (mentioned above) and make sure you understand how the enrollment rules work and what that means in your particular situation. If you have questions about enrollment in Medicare Part A or/and Part B, contact Social Security at 1-800-772-1213 from 7 a.m. through 7 p.m. (whatever time zone you are in) to ask your questions. You should also read Social Security’s info about enrollment in Part C and Part D if you need information about that: https://www.medicare.gov/Pubs/pdf/11219-Understanding-Medicare-Part-C-D.pdf.
Medicare is a wide-ranging medical insurance program. You should start trying to learn about it and how it will work in your situation before age 65. Retirement is like a trip to a new place, check the roadmap before you start.