Although the summer months see peak vacation travel across the US, temperatures will depend on region, altitude and latitude; typically, the Rockies offer more temperate climes compared to the vast landscapes of the plains. The Grand Canyon and the mighty national parks (Bryce, Yosemite, Yellowstone etc.) are great to visit in May and June. Montana, Wyoming and Utah are cooler than SW USA (Arizona, Nevada, New Mexico and Texas). The California coast enjoys the most temperate climate even in peak summer.
The spring and autumn months are the best time for a vacation to all areas that sizzle in summer, viz. the low-lying plains of SW USA (Arizona, Nevada, New Mexico, Texas). Yosemite and Yellowstone are gorgeous in the fall, with mild climates, fewer crowds and barely any rainfall. All in all, if you aren’t vacationing up in the mountains (a bit chilly in these shoulder months), and you prefer lesser crowds, spring and autumn are the ideal and most pleasant months for travel all across the west and mid-west.
Except for California, which enjoys a temperate climate year-round and the ski-slopes of Colorado (peak season), winters all over the mid-west and western USA can be pretty harsh, with plummeting temperatures and bone-chilling winds. The Pacific Coast sees better temperatures than inland, but definitely avoid the Portland and Washington State, where there is near-continuous rainfall through January and February.