In light of recent events, this is a revision of a previous post on the best time to visit. Disregard this post if you aren't in the process of planning a vacation to SD.
Comments on weather throughout the year:
*November-April: Avoid unless you want to ski or do some snowmobiling. It gets dark early. Many attractions and businesses in the smaller towns are closed. Roads, especially those at higher elevations, may close due to snow. Interstate highways and airports close during blizzards. Power outages happen during blizzards. Temperatures (especially Dec-Mar) may drop below zero degrees F. If you must visit during this time, rent a four wheel drive vehicle and keep emergency supplies, such as boots and a shovel in the vehicle with you. For anyone coming from the south or warmer countries, mittens and a hat that covers your ears are important items to wear. Check weather forecasts before setting out on any day trips. Freak blizzards happen. If you get caught in a blizzard, stay in your vehicle until help arrives. People who wander out in blizzards to find help get lost and disoriented. It is also suggested that you stay in Rapid City if you come at this time of year. Rapid City businesses will be open year-round.
*May and October: In light of recent events, it is now suggested that you avoid coming in these months. You will be gambling with your hard-earned vacation money. On the one hand, the weather could be fabulous. You might see 70 degree temperatures and sunny days. On the other hand, you could encounter snow and blizzards. Note that on October 4, 2013, Lead, SD saw 43 inches of snow fall and Rapid City had 20 inches. 4,000 customers were without power. Interstate 90 closed from Murdo, SD to the Wyoming state line. Flights were cancelled at the Rapid City airport. While this type of blizzard in October is rare, it is possible. Out-of-control weather seems to be getting more and more common. That being said, May and October are still better choices than Nov-April, but be warned you are taking a risk. If you must visit during this time of year, please note that touristy attractions may not be open. Phone ahead before visiting to see what's open. Storybook Island, Old McDonald's Petting Zoo, the 1880 Train and the Alpine Slide are among the attractions that close or have reduced hours in October.
*June: This is a good time to visit, especially mid-late June. The first two weeks of June often have cool, rainy weather. Some people may actually prefer this cooler weather for hiking. This is not a beach vacation, so there will be plenty of things to do even if the weather is on the cool side. Forest fire season does not peak until later in the summer. Early June may see reduced pre-season rates. Around the 15th of June, the weather usually warms up and the end of June can even get hot.
*July: The first three weeks in July are a good time to visit. The weather may be hot and you could see temps surpass 100 degrees. Despite this, many people do not mind the heat and this is a very popular time to visit.
*The last few days of July, the first two weeks of August: Avoid coming at this time unless you want to attend the Harley Davidson rally. This annual Sturgis event attracts thousands of bikers from all over the world. Lodging prices will be high and lodging may be difficult to find. You will not be able to avoid bikers by staying in other towns as they will be all over the Hills, at the Badlands and at Devil's Tower. Riders seem to be arriving earlier and staying later each year, so crowds aren't necessarily confined to the "official" rally dates.
*The end of August: This is also a great time to visit. Temps may still be hot and might surpass 100 degrees. If you have children attending schools in other states, this is a good time of year to visit because most SD schools start around August 20th, give or take a few days, and this reduces the number of tourists.
*September: If you don't have children in school, regulars on this forum consider this the best time possible to visit. The sooner after Labor Day, the better, as it will start getting cool by the end of the month. The weather in September is usually not too hot or cold. A few touristy attractions may close after Labor Day, but the major scenic attractions will be open. Some people enjoy attending the annual buffalo roundup in Custer State Park. This is something you may really want to see or really want to avoid because it does attract several thousand people.