Assuming you are asking about the best time to visit Yellowstone National Park in terms of weather, the answer would be June, July or August. These months offer the warmest temperatures and longest days, making them ideal for exploring all that the park has to offer. However, it should be noted that these months also tend to be the busiest, so if you are looking for a less crowded experience, you may want to consider visiting Yellowstone in May or September instead.