The best months to camp in Yellowstone National Park are from June to September, when the weather is mild and most park facilities and campgrounds are open. July and August are the peak months for visiting the park, with pleasant daytime temperatures averaging 70-80°F (21-27°C). However, these months also see the highest influx of visitors, which can result in crowded campgrounds and attractions. For a less crowded experience, consider visiting in June or September, but be prepared for cooler temperatures, especially at night. Keep in mind that some campgrounds and facilities may close after Labor Day in September.