The Initial Enrollment period is a 7 month period starting 3 months before your birthday month in which you turn 65. It includes your birthday month and the 3 months after.
The Genreal Enrollment period is every year from January 1-March 31. Your coverage will not start until July 1.
Special Enrollment is for people who didn't sign up when first eligible due to group health coverage from an employer (your own or a spouse or family member).
You can sign up-anytime you are still covered by the group health plan or during the 8 month period after the coverage ends.