iRacing uses seasons to organize their online competition. Each calendar year features 4 seasons, which makes each season 13 weeks per season.
Each season iRacing releases a schedule for each of the series that is offered through the service. This usually consists of the first 12 weeks which are considered normal events. The 13th week is one without official races and instead features a variety of fun events. These usually consist of more exotic track & car combinations. Learn more about iRacing's Week 13.
When seasons roll over iRacing usually releases a major patch. Big patches during the first 12 weeks of any season are rare and are usually reserved for smaller patches that contain bugfixes for the latest major release.
During each season a player can compete in a number of events. At the end of the season the iRacing service will review your performance to see if you are eligible for a license promotion, if you did well, or even a demotion if your performance (or safety) wasn't up to snuff.