The standard Open Enrollment for health insurance will start November 1st 2021 and run until December 15th for plans starting January 1st, 2022.
For context, each year the government sets aside a specific time called Open Enrollment period (typically November 1st to mid-December) where anyone can purchase a new insurance plan for the coming 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 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 medial 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 has losing job based coverage or having a baby.