Preseason
The preseason exists in every league in OOTP. In leagues with Spring Training, the preseason starts 30 days prior to Spring Training. In leagues without Spring Training, the preseason begins 60 days before Opening Day.
The one exception is the first year of a game:
Standard Game starts around March 18th just before Spring Training games start.
Historic Leagues begin March 30th just before Opening Day.
Custom Fictional Game you can set when you want to start your season from January 7th up to December 31st of the year you are in. NOTE: Trying to start a custom game before January 7th will cause game to put up and error and go back to the game start screen.
Games start this way to allow players to make adjustments to their league settings immediately after starting their games.
There are no specific tasks that need to be handled by the GM during the preseason. However, many league settings can only be customized during the preseason or Spring Training, so if you're looking to make changes to your league's rules or settings, you should consider doing it during the preseason.
Note: In some leagues you might have the option to run some short programs in the Development Lab before a season starts.