If you miss open enrollment and need health insurance, you'll need to qualify for "special enrollment" by having a qualifying life event to apply. The following list includes the most common qualifying life events:
Losing existing health coverage
Turning 26 and losing coverage through a parent's plan
Getting married or divorced
Having (or adopting) a baby
Death in the family
Moving to a different ZIP code or county
Becoming a US citizen
Please see our article on What is a Qualified Life Event (QLE) if you are outside of open enrollment, and wish to shop.