You can sign up for "premium-free" Part A (Hospital Insurance) any time because you or your spouse (living, deceased or divorced) paid Medicare taxes during at least 10 years of work. However, you can only sign up for Part B (Medical Insurance) during the General Enrollment Period between January 1 - March 31 each year. Your Part B coverage will start the month after you sign up.
You may have to pay a late enrollment penalty for not enrolling in Part B when you were first eligible. Learn more about how to sign up and the late enrollment penalty.