Summer is the obvious season: warm dry days in the seventies and eighties, cool nights, the McCloud River trails fully open, and the lower falls swimmable. July and August are the peak. May and June are quieter and the wildflowers are out, but the upper sections of the falls trail can still hold snow.
Winter draws a different crowd to the Mount Shasta Ski Park just up the road. December through March is the season, with reliable snow on the upper slopes. Spring and autumn are the in-between months — fine weather, very few visitors, and the falls trail at its most peaceful. Storms in late autumn can dump enormous amounts of snow on the higher passes; check road conditions before driving.