Every year the General Enrollment Period (GEP) for Medicare occurs between January 1 and March 31. This is the time—and usually the only time—during which people who are eligible for Medicare Parts A and/or B but did not do so when first eligible have another opportunity to join. 

Generally, people are first eligible to enroll in Medicare during their Initial Enrollment Period. This period starts either three months before you reach your 65th birthday or after two years if you have received Social Security Disability Insurance (SSDI) for 24 months prior to age 65 . 

During the Initial Enrollment Period, most people who are still actively employed past their 65th birthday and get their health insurance through their employer (or an actively working spouse) can safely delay enrolling in Medicare Part B until the worker retires. However, people who don't sign up for Medicare Parts A & B on time and don't have other insurance through active employment may face a late enrollment penalty.

As this fact sheet explains, you may use the Medicare GEP to enroll at any time during January, February, or March. Regardless of when you enroll and which parts of Medicare you choose, however, all coverage will start on July 1. That’s one of the many reasons why it’s so important to sign up for Medicare when first entitled to do so.