Open enrollment is the time period to choose or change health insurance plans. For plans under the Affordable Care Act, open enrollment starts on Nov. 1, 2017, and ends Dec. 15, 2017. Medicare open enrollment runs from October 15 thru December 7, 2017. Coverage for both the ACA plans and Medicare starts on Jan. 1, 2018. If your employer offers health insurance, be sure to check with your human resources department because employer-based coverage periods vary.