According to Dan Rafael via ESPN.com, Cotto-Canelo will fight this October or November. They would prefer to go on September 12th, which is of course the weekend of the Mexican Independence Day.” Dan Rafael.
Canelo would prefer to fight in September around the Mexican holiday however Mayweather has recently requested September 12th @ the MGM for a PPV event on Showtime. This is also Mayweather's last fight with Showtime and as he has stated, its his last fight before he retires.
HBO and the promoters don’t want to compete with Mayweather to follow another PPV the next week.
The Cotto vs. Canelo fight promises to be an excellent mouthwatering bout that boxing fans have wanted to see for some time. So both camps are going to wait until the official announcement of the Mayweather fight then figure out the details of the venue and dates.