Every year from October 15 through December 7, anyone on Medicare can change their Medicare Advantage or Part D plan. Changes made in that window take effect the following January 1.
Plans are allowed to change their costs, their networks and their drug lists every January. The plan you chose is not the plan you will have next year unless you check. Every fall your plan mails you an Annual Notice of Change, and it is worth actually opening.
It is not the window for a Medicare Supplement, which works on different rules and can be applied for at other times, generally with health questions. And it is not the only window that exists. Turning 65, moving, losing employer coverage, and having both Medicare and Medicaid can each open one at other times of year.
Do not wait until the first week of December. The last days of the window are the busiest of the year, and a rushed decision about your drug coverage is how people end up paying more for the same prescriptions.
General information only goes so far. What is actually offered where you live, and whether it fits your doctors and prescriptions, takes about ten minutes to check. No charge for the review.