Special Enrollment Periods are granted for a number of reasons:
Persons new to Medicare, either turning 65 or receiving disability. You may join a plan 3 months before you receive Medicare, the first month of coverage, and 3 months after. That is a 7 month window to enroll in a plan. The earliest prescription coverage can begin is the first day of the month your Medicare part A (Hospital) and/or B (Medical) begin.
Person whose Creditable Cover ends. A person who is losing their current coverage because they are retiring or because the employer is no longer offering prescription drug coverage will not have a penalty added to their monthly premium in this situation.
Other reasons for a Special Enrollment Period include: moving in/out of a nursing home or loss of Low Income Subsidy (Extra Help).