The "pre-season started one day early" bug is actually an un-fixed bug for a good three years.
Before the introduction of the Association Mode, there was only one game in the final day of the season, and it worked okay. It was only an inconvenience to not be able to see the "next opponent" after the last game on the second last day of the season.
But now, since there are three games on the final day of the season, not being able to see any of the three "next opponent" has become a bug that breaks the game on the last day of the season. And not being able to view nor manage both the association games pretty much broke the association mode.