When should I visit American Southwest?
I would recommend you avoid hot summer months as the temperature rises to 100+ degrees (40+ C) and to be honest hiking in that weather is not enjoyable and even dangerous.
The best times to visit Utah and Arizona are April and October and November. Those periods are snow free but not too hot.
Go check out my guide for the best free things to do as well as itineraries and travel tips to make your trip unforgettable.