 There are over a dozen such enrollment periods.In addition, a special Medicare Advantage Open Enrollment period extends from January to March of each year during which the over 20,000,000 people on Medicare Advantage (but not others on Part C) can switch or drop plans.This January-March enrollment period is not to be confused with the annual election period, which was intended initially primarily for Part D of Medicare, because the January-March Open Enrollment applies only to people already on Medicare Advantage.