There are three different Medicare open enrollment periods to begin your Medicare, called your Initial Enrollment Period (IEP), Special Enrollment Period (SEP), and the General Enrollment Period (GEP). When you apply for Medicare determines which Medicare enrollment period you fall into. It is important to understand the different enrollment periods, because it determines when your Medicare Part B will begin. 

The is also an Medicare enrollment period called the Annual Election Period, also known as Medicare Open Enrollment, where you can make changes to your current Medicare coverage.