An Introduction to Medicare: Parts A, B, C, and D Explained

August 24, 2025 · Personal Finance

Key Enrollment Periods: When to Sign Up

Timing is everything with Medicare. Enrolling at the right time ensures your coverage starts when you need it and helps you avoid lifelong penalties. There are several specific windows of time, called enrollment periods, for signing up.

Initial Enrollment Period (IEP): This is your first and most important chance to sign up. It is a 7-month period that starts 3 months before your 65th birthday month, includes your birthday month, and ends 3 months after. It is the ideal time to enroll in Parts A and B, and also to choose a Part C or Part D plan.

Special Enrollment Period (SEP): If you or your spouse are still working past 65 and have health coverage through that current job, you may be able to delay enrolling in Part B without penalty. When that employer coverage ends, you will get an 8-month Special Enrollment Period to sign up for Part B.

General Enrollment Period (GEP): If you missed your IEP and do not qualify for an SEP, you can sign up for Parts A and B during the General Enrollment Period, which runs from January 1 to March 31 each year. Your coverage would then start on July 1. You may have to pay a late enrollment penalty if you use this period.

Annual Open Enrollment Period (AEP): This is your yearly chance to review and change your coverage. It runs from October 15 to December 7. During this time, you can switch from Original Medicare to a Medicare Advantage plan, switch from one Medicare Advantage plan to another, or join, drop, or switch a Part D prescription drug plan. Changes you make take effect on January 1 of the following year.

Medicare Advantage Open Enrollment Period: This runs from January 1 to March 31. If you are already in a Medicare Advantage plan, this period allows you to switch to a different Medicare Advantage plan or switch back to Original Medicare (and select a Part D plan).

Keeping these dates in mind is a critical part of managing your healthcare. Mark your calendar for your Initial Enrollment Period—it is the smoothest on-ramp to your Medicare coverage.