I lived in Atlanta from 2001 to 2012.
Labor Day weekend usually included multiple football games (Georgia Tech, Georgia State more recently, Chic-Fil-A Kickoff Classic, Atlanta Falcons), Braves' games, Music Midtown, and occasionally a pride festival. It was easily the biggest weekend of the year in Atlanta and downtown and midtown are usually packed.
As others have noted, DragonCon is huge, too.