Picking a games release date is tricky business. A lot of huge games generally don't get released during the summer because there are not big holidays to push sales, but then again releasing during the summer for the not-so-huge titles can be rewarding since they won't have to go against the blockbuster.
For the blockbusters, they have to pay attention to when the other huge games are being released. Releasing a week before or after those big releases could mean death to your sales since people will be saving to buy that huge game.
There's also the idea of releasing your sellers on or around the same time as your competition to put a hickup in their sales. Just look at September 25th for example: Halo 3, Jericho, and Assassin's Creed. Also, look at the week before and after September 25th--no big releases there (Except Eternal Sonata, but that's a completely different target audience).
