December 15 is the last day you can enroll for health insurance and be covered January 1!
If you miss this deadline, you will not have health insurance when the calendar turns over. And you'll have to go without health insurance until the first of the following month.
So if you or a family member needs to visit a doctor or fill a prescription during that time, you will have to pay the entire bill out of your own pocket. Are you able to shoulder that potentially heavy expense?