Spring and autumn are the best times to visit Grass Valley. April through June brings mild temperatures, wildflowers on the surrounding hills, and manageable crowds at the Empire Mine. September and October offer warm days, cool evenings, and the Nevada County harvest festivals that take advantage of the area's wine and farm culture.
Summer is warm but not punishing — elevation keeps temperatures more comfortable than the Central Valley below. Winter brings occasional snow, which makes the Victorian downtown look properly cinematic, and the ski resorts at Donner Summit are about an hour's drive. The Cornish Christmas celebration in December is a long-standing local tradition with caroling and market stalls on Mill Street.