You can join a Medicare Part D or Medicare Advantage plan from October 15 - December 7
of each year.
In most cases, if you don't join by December 7, and you don't currently have a drug plan that, on average, covers at least as much as standard Medicare prescription drug coverage, you will have to wait until October 15 of the following year to join and may incur a permanent late enrollment penalty
When you do join, your premium cost will go up at least 1% per month for every month that you wait to join. Like other insurance, you must pay this penalty as long as you have Medicare prescription drug coverage.
If you join by December 7th, your coverage will begin January 1.
(Source: U.S. Department of Health & Human Services)
You can see more on the annual Open Enrollment Period or AEP