In most states, the Open Enrollment period for health insurance will start November 1st, 2021 and will run until January 15th, 2022. To secure coverage starting January 1st, 2022, you must apply and pick your plan by December 15th, 2021.
Note, in some states the deadline has been extended past January 15th, 2022. View those states here.
For context, each year the government sets aside a specific time called Open Enrollment where anyone can purchase a new insurance plan for the upcoming year.
The government created this time because insurance works by combining the money that people pay each month in premiums to cover medical costs. Without the Open Enrollment Period, many people would only apply for insurance once they got sick or required expensive medical care, and there would not be enough funds in the system to cover everyone’s medical costs.
Outside of Open Enrollment, people may qualify for a Special Enrollment Period. Usually to qualify during this time you'd need a qualifying life event, such as losing job based coverage or having a baby.