Open season is also known as open or annual enrollment. It is a set period of time that typically happens once a year, that allows insured individuals and companies to make changes to their existing health insurance. This is also the period of time during which Americans may buy health insurance through online health insurance exchanges.
In terms of Medicare, open enrollment is when close to 50 million insureds may opt to stay in original Medicare or change plans within Medicare Advantage and/or Medicare Part D prescription drug programs for the next year. Individuals can only make changes to their insurance or extra benefits during open enrollment, or if they have a specific qualifying event.