As a former resident of Canton, OH, I can tell you that they usually announce the teams playing in the Hall of Fame game about a month after the Super Bowl is played. It is always the first game of the preseason (except for preseason games that sometimes happen in overseas), at least it's supposed to be the "first official" preseason game of the year.
It's also always a boring game because the starters only play the first series or two. But it's cool to be at the game because the stadium is more of a high-school stadium than a pro stadium, so even in the "bad" seats, you get closer to the players than you would in some of the best seats at Texas Stadium.