You usually become eligible for Original Medicare around your 65th birthday, and as you approach that landmark date, you will enter your Initial Enrollment Period (IEP). This is a seven-month period that: Begins the first day of the month three months before the month of your birthday. If your birthday is in October, then your IEP begins July 1.; Ends the last day of the month three months after your IEP. If your birthday is in October, then then your IEP ends the following Jan. 31.