Medicare open enrollment runs from October 15 to December 7, allowing beneficiaries to switch between original Medicare, Medicare Advantage, and Part D plans, with changes effective January 1. If no action is taken, coverage renews automatically, which may be costly if plan details change. A second enrollment period from January 1 to March 31 lets Advantage enrollees switch plans or return to original Medicare, though obtaining Medigap coverage may be difficult. Using Medicare's Plan Finder helps compare plans and save on costs.