The “Initial Enrollment Period” (IEP) is around your 65th birthday (± 6 months) for Parts A & B. If you delay enrolling in Part B and you don’t have other creditable coverage, you may face a penalty. 
The Annual Election Period (AEP) (also called Open Enrollment) runs October 15 to December 7 each year. During this time you can:
Switch from Original Medicare to a Medicare Advantage plan (Part C)
Change Medicare Advantage plans
Join or switch a prescription drug plan (Part D)
There are “Special Enrollment Periods” (SEPs) if you have life changes (e.g., move, lose employer coverage, etc.).