Seniors can find open enrollment dates for Medicare Parts C and D on the Medicare online tip sheet entitled "Understanding Medicare Part C & D Enrollment Periods." The open enrollment period for Medicare Part C and D is Oct. 15 through Dec. 7 each year.
Because all seniors are automatically enrolled in Medicare Part A and B when they begin collecting Social Security, most seniors do not participate in the open enrollment periods for Part A and B, according to Medicare. Seniors who are older than 65 but are still working and not collecting Social Security are not automatically enrolled in Part A and B as of 2015. Employed seniors can still apply for coverage at SocialSecurity.gov or by visiting or calling a Social Security office, notes Medicare. The special enrollment period for employed seniors begins three months prior to turning 65 and extends three months after turning 65. The General Enrollment period for seniors who became eligible for Medicare but were not automatically enrolled and did not apply during their special enrollment period is Jan. 1 through March 31 each year.