To enroll for Medicare, there are 2 periods you can enroll in:
Open Enrollment Period: From October 15th to December 7th each year. You can join, switch, or drop a plan. Your coverage will begin on January 1 (as long as the plan gets your request by December 7).
Medicare Advantage Open Enrollment Period: From January 1st to March 31st each year. If you’re enrolled in a Medicare Advantage Plan, you can switch to a different Medicare Advantage Plan or switch to Original Medicare (and join a separate Medicare drug plan) once during this time.

A lot of questions get asked concerning enrollment for Medicare, read this blog and get answers to those questions you've been confused about.

Click the link in the bio to learn more.
https://www.medicare.gov/sign-up-change-plans/joining-a-health-or-drug-plan