Summer in the Park
A summer visit can be a really great time to get out and explore. The summer season runs from about mid-April to the beginning of November. The peak months for the park are July and August. There are more people that visit the park as the weather gets warmer.
This season is incredible for hiking, camping, and biking but you have to beware of bears and wolves the most at this particular time. Site seeing in the summer can be beautiful and the weather is very enjoyable.
Fall in the Park
A fall visit can be unpredictable and roads can be closed because of snow or weather conditions. If you are thinking about visiting the park in the fall, the National Park site suggests snow tires or chains depending on the weather.
I would not suggest visiting the park at this time if you are wanting to see the park as a whole. It may be a good time to lodge but not to drive around the park.
Winter in the Park
A winter visit to Yellowstone encompasses steaming geyser basins and snow covered landscapes as well as some incredible wildlife viewing. During the winter the park offers cross-country skiing trails.
In the winter the lodges are open. The lodge near Old Faithful is the most popular to stay at in the winter.
Spring in the Park
A spring visit is the quietest time to visit the park. There are not many services available between mid-March and the beginning of May but this time of year can still be fun for some.
Spring skiing is popular for many people visiting the park at this time of year. As well as skiing, the Northern roads are also plowed for travel. Although this is a quiet time of year, it is still cold outside. If you are prepared for the cold then enjoying the wildlife and incredible scenery can be breathtaking.