Medicare open enrollment runs from October 15 to December 7, allowing enrollment or plan changes, including switching between original Medicare and Medicare Advantage. Original Medicare (Part A and B) holders can buy Medigap policies to cover out-of-pocket costs, but only within a six-month Medigap Open Enrollment period starting with Part B enrollment. Medigap plans vary by state and coverage, with Plan G being most popular. Medigap does not cover prescription drugs, long-term care, or vision.