Is there a sports fan convention today? What characters are people cosplaying? I keep hearing about it, how big is it compared to other sports conventions?
Sports fandom is quite varied. At any given moment in time you'll find fans of all types from different walks of life and backgrounds united by their love for the sport they follow. 
Since most major sporting events happen at certain times of year, many conventions occur around these occasions as well. For example: 
1. Super Bowl (February)
2. NBA All Star Game (February or March)
3. Major League Baseball Playoffs(August - November).