If you want coverage that meets minimum ACA standards, you have to sign up during the annual open enrollment unless you qualify for a special enrollment period. For 2019 coverage, the open enrollment period for marketplaces begins November 1, 2018 and extends through December 15, 2018. Some states have established longer open enrollment periods, so check with your marketplace. Once the open enrollment period is over, individuals and families will not be able to enroll in marketplace health plans until the next open enrollment period. However, if you experience certain changes in circumstances during the year, you may have a special 60-day opportunity to enroll in marketplace health plans, through a special enrollment period. (45 C.F.R. §§ 155.410, 155.420).
Individuals and families buying non-group coverage on their own, outside of the marketplace, can also only enroll in coverage during open enrollment periods and special enrollment opportunities. (45 C.F.R. § 147.104).
American Indians and Alaska Natives can enroll in coverage throughout the year, not just during open enrollment periods or via special enrollment opportunities. (45 C.F.R. § 155.420; HealthCare.gov, Health coverage for American Indians & Alaska Natives).