The high season to visit Yellowstone National Park is summer. The months of July and August are the most popular. The weather is warm and dry, and this is a perfect time for enjoying nature and watching wildlife.
Although you should expect large crowds, due to summer holidays at schools. Expect heavy traffic on the roads of the park and even traffic jams. Nights are also warm and lots of visitors stay in the campgrounds. This is also good time for various outdoor activities.