When you first get Medicare
When you're first eligible for Medicare, you have a 7-month Initial Enrollment Period to sign up for Part A and/or Part B.
For example, if you're eligible when you turn 65, you can sign up during the 7-month period that begins 3 months before the month you turn 65, includes the month you turn 65, and ends 3 months after the month you turn 65.
Between January 1–March 31 each year
If you didn't sign up for Part A and/or Part B (for which you must pay premiums) when you were first eligible, and you aren’t eligible for a Special Enrollment Period (see below), you can sign up during the General Enrollment Period between January 1–March 31 each year.
Special circumstances (Special Enrollment Periods)
Once your Initial Enrollment Period ends, you may have the chance to sign up for Medicare during a Special Enrollment Period. If you're covered under a group health plan based on current employment, you have a Special Enrollment Period to sign up for Part A and/or Part B any time as long as you or your spouse (or family member if you're disabled) is working, and you're covered by a group health plan through the employer or union based on that work.
You also have an 8-month Special Enrollment Period to sign up for Part A and/or Part B that starts the month after the employment ends or the group health plan insurance based on current employment ends, whichever happens first.
Usually, you don't pay a late enrollment penalty if you sign up during a Special Enrollment Period.
COBRA and retiree health plans aren't considered coverage based on current employment. You're not eligible for a Special Enrollment Period when that coverage ends. This Special Enrollment Period also doesn't apply to people with End-Stage Renal Disease (ESRD).
You may also qualify for a Special Enrollment Period for Part A and Part B if you're a volunteer, serving in a foreign country.