Fall is the best time to visit, and the crowds know it. October brings spectacular foliage to the Smokies — the ridgelines turn amber and crimson from mid-October through early November — but also the heaviest tourist traffic of the year. If you want the fall colors without the gridlock, aim for a weekday in late October or come the first two weeks of November when the leaves are past peak and the crowds have thinned.
Spring is arguably the second-best season: wildflowers bloom throughout the park from March through May, temperatures are mild, and the summer hordes haven't arrived. Summer is hot, humid, and packed — school vacation season brings the heaviest traffic, and parking becomes genuinely difficult. Winter is quiet, occasionally cold enough for snow on the higher elevations, and certain weekends around the Winterfest lights display (November through February) draw their own crowds.
Gatlinburg sits at around 1,200 feet elevation, so summers are somewhat cooler than the surrounding lowlands. The Smokies above the town can be significantly cooler — bring a layer even in July if you're hiking at altitude.