Special Enrollment Period

A set time when you can sign up for Medicare Part B if you didn't take Medicare Part B during the Initial Enrollment Period, because your or your spouse were working and had group health plan coverage through the employer or union. You can sign up at anytime you are covered under the group plan based on current employment status. The last eight months of the Special Enrollment Period starts the month after the employment ends or the group health coverage ends, whichever comes first.

Source: Medicare.gov