In general, you can only enroll in non-group health plan coverage during the open enrollment period. For 2018 coverage, the open enrollment period for marketplaces begins November 1, 2017 and extends through December 15, 2017. 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).