The windows, in the order most people meet them.
Initial Enrollment Period. Seven months: the three before your 65th birthday month, that month, and the three after. Enroll in the early months and coverage starts the month you turn 65. This is the window that matters most.
Special Enrollment Periods. Working past 65 with coverage from a large employer lets you delay Part B penalty-free, then enroll during the months after employment or that coverage ends. COBRA and retiree coverage do not count as employer coverage for this purpose, a distinction that catches people every year. Other special periods follow moves, plan closures and similar events.
General Enrollment. January through March each year, for people who missed the windows above. Coverage starts the month after enrolling, and late penalties usually apply.
Medigap open enrollment. The six months after Part B starts. Insurers must accept you regardless of health during it, and in most states never have to again. One-time, per person.
Annual Open Enrollment. October 15 to December 7, every year, for switching Medicare Advantage and Part D plans, effective January 1. A separate Advantage-only window runs January through March for people already in an Advantage plan.
The penalties, briefly: Part B late enrollment adds 10 percent to the premium for each full year late, permanently. Part D adds a smaller monthly amount for each month without creditable drug coverage, also permanently.
Exact dates and personal eligibility are at Medicare.gov.