Even better than preseason, if the Red Sox made the World Series, you could have Pats/Celtics/Bruins regular season games, and a World Series game on the same day.
2009:
World Series Game 4 was on 11/1/09
Celtics beat the Hornets on 11/1/09 (starting off 4-0)
Bruins lost to the Rangers on 11/1/09
But Patriots had an off week.
That could have been it right there.
Though it would be tough. Red Sox would have to go all the way to the World Series and Bruins rarely play Sunday games.