When can I join a Medicare drug plan?

There are specific times when you can sign up for these plans, or make changes to coverage you already have. You don’t need to sign up for Medicare each year. However, each year you’ll have a chance to review your coverage and change plans.

I'm going on Medicare A and B for the first time (Initial Enrollment Periods for Part C & Part D)
Are there certain times each year I could obtain new, or change existing drug coverage? (Yearly enrollment periods for Part C & Part D)
Special Circumstances (Special Enrollment Periods)