Short answer: Your first chance is the 7-month Initial Enrollment Period around your 65th birthday. After that, the Annual Enrollment Period (Oct 15 to Dec 7) lets you change Medicare Advantage and Part D plans, and Special Enrollment Periods apply after events like leaving employer coverage.
Medicare has several enrollment windows. The Initial Enrollment Period spans the 7 months around your 65th birthday (three months before, your birthday month, and three months after). The Annual Enrollment Period (October 15 to December 7) is when anyone can join, switch, or drop Medicare Advantage and Part D plans for the next year. There’s also a Medicare Advantage Open Enrollment Period (Jan 1 to Mar 31) for those already in an Advantage plan. Special Enrollment Periods open after qualifying events; most importantly, losing employer coverage when you retire, which lets you enroll in Part B without the late penalty. Missing your window can mean lifelong late-enrollment penalties.