Deadwood is busiest from June through August, when the air is warm, the saloon re-enactments run daily and the surrounding Black Hills are open for hiking and driving. The Days of '76 Rodeo in late July and the Sturgis Motorcycle Rally in early August both bring massive crowds — rooms book out months in advance and prices double, which is fine if you're there for the noise and terrible for anything else.
Late September and October are the best time to visit: smaller crowds, aspens turning yellow on the hillsides, and most attractions still open. Winters are cold and snowy, and many of the smaller museums close, though the casinos stay open and the Christmas decorations on Main Street are genuinely charming.