Most people who already have Medicare can switch Part D or Medicare Advantage plans from October 15 to December 7. Coverage starts January 1. That window is the Annual Enrollment Period. It is not the only door, but it is the one that does not require a story.
If you are new to Medicare, your Initial Enrollment Period is a seven-month window around your 65th birthday month. Missing it can mean a late-enrollment penalty that lasts as long as you have Part D—unless Extra Help later wipes it out.
Special Enrollment Periods exist when you move out of a plan’s area, lose other creditable coverage, enter or leave an institution, or qualify for Extra Help or a Medicare Savings Program. Extra Help generally lets you change drug plans once in January–March, once in April–June, once in July–September, and again during the fall AEP. Do not wait for October if a formulary change in March is going to break an expensive fill.
Enroll on Medicare.gov or by calling 1-800-MEDICARE. A plan’s 800 number will enroll you in that plan. A television number may enroll you in something else. ThriftMeds can store a demo enrollment on your dashboard so you can practice the steps. It does not send an application to CMS. When you are ready for the real thing, take the plan name, the contract and plan IDs, and your medication list to Medicare.gov.