The going to the sun road doesn't usually open until at least the third week of June. May and June can be wet and rainy, so trails can be washed out so you may need special gear for hiking, although it makes for great rafting. July and August are usually dry, but can also be very hot so always have plenty of water. September can be great, if it doesn't snow.
July and August are the best times. Though the weather may be sunny and mild but it can change rapidly so, make sure you have some thing to keep you warm.
My preference is cooler weather and October is perfect for 40-50 degree days. Its off season and although the Sun Road was closed there is still access to many parts of the park, and attendance is light freeing up parking and the trail hikes.
Based on the weather and snow removal and snow melt, usually opens june or July and closes sometime in September and October. There is no set date because of weather.
Just be prepared wether is in predict so it can be hot and two hours later it can be raining. we have been there several times it rain always some times is cold.
Summer is crowded but mid-july through Labor Day is beautiful
July 15 to August 21
August for sure.
Mid July to September
August is a good time
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