The National Monuments and National Parks (including Mount Rushmore ) are open year-round. In old-fashioned vacation style, the Black Hills  of South Dakota are fully open and ready for business between Memorial Day and Labor Day, which constitutes the high season for tourism.
In the “shoulder season” (from May 1 to Memorial Day, and from Labor Day to mid-October), most attractions are open and there is still plenty to do. If the purpose of your visit is primarily scenic and recreational in nature, there are hotels open year-round in every region.
The weather in the hills is unpredictable, but spring is the season where cooler temperatures and rain are most likely. It is also the season when the region is at its greenest. Early summer tends to be warm and dry and brief afternoon thundershowers are not uncommon.
Mid- and late summer can be anywhere from comfortable to extremely hot, and temperatures vary greatly between the warmer Southern Hills  and the cooler mountain towns. For travelers not tied to a school schedule, early fall can be the best season to travel, with warm days and cool nights. It is my favorite season here.
The Sturgis Motorcycle Rally  begins on the first Saturday in August and lasts a week. During the rally, hundreds of thousands of bikers flock to the hills and fill the campgrounds and hotels. Traffic congestion in the parks and on the roads is common during the rally and room rates are at their highest.
For some, it’s the perfect week to come, but if the constant roar of motorcycles is not for you, it might be a week to avoid. If you’ve come to the hills inadvertently during the rally, rooms are generally available and the environment is a little quieter in the Southern Hills, particularly in Hot Springs .