iRacing has four Seasons each year. Each season consists out of 12 weeks with serious racing. When we say serious, we mean the type of racing that matter when it comes to your iRating and division. Week 13 isn't listed on these schedules, but rest assured you can count on it four times a year!
During week 13 there are less events scheduled, yet those that are scheduled are more focused on having fun. Think of crazy track & car combinations and opening up a ton of content for everyone to enjoy.
During the Week 13 events there are a number of events that do not count towards your Safety Rating or iRating. However, there will be two official series (road & oval) that are open to everyone to enjoy, as there is no license requirement. This is a great way to explore different content, but do be mindful that these races can be a bit chaotic. One of the reasons is that instead of having a full week to practice and race, there's a new race every day (see below). Beyond that, since there is no real license needed the actual requirement to join is low, which also means less experienced drivers get mixed in - which can create more uncertainty and unpredectability.
During the 13th week it's almost like a super season as there's a new track and car combo every day for one week straight. These two these official series count towards your iRating and Safety Rating.
Although the sim racers might think it is to have a blast or have a week off (which are both great reasons), the actual reasoning behind week 13 is that it allows the iRacing team to roll out the new patch and additional changes in a week that does not have such a big impact (as it does not count towards your championship points and particiaption). Essentially it's a test week or test server where we can all have fun.
"It’s our opportunity once every few months to throw some fun series out for you to take a joyride, while selected official series and races continue around our scheduled service downtimes to implement new content and updates to the iRacing service."iRacing.com - Week 13 is coming
So now you know that whenever people refer to Week 13 they mean the end of the season which could also be seen as an off week for those that are running full seasons as they can decide not to compete without suffering any penalty or not earning points. This makes the week overall less serious as there essentially is nothing to gain or lose, besides having a blast on the track. The iRacing team will test the new patch and changes during this week.