The USA is a very big country to visit, so choose carefully the places to see and the time to go. Bring a good map because you may want to travel around.
New York and Washing D.C. are good places to visit in May or October. It’s not too hot then. In winter there’s a lot of snow.
The best plan is to arrive in New England in September. The weather starts to get cooler and the trees starts to change color. It’s good idea to bring your camera because you may want to take photos of the autumn leaves.
4,000 kilometres away in Los Angles, California, the weather is fine all year. It’s nice to see the sun in December! Don’t forget to bring your swimsuit because you might want to swim in the sea.
In the northwest, it isn’t very cold, but there’s a lot of rain, so bring an umbrella. It’s pleasant to visit Alaska in July and August but it might be cool in the evening. So remember to wear some warm clothes. But don’t come in winter. It’ll dark all day and freezing cold. 
In Texas and the southeast, there are storms from time to time in summer and fall. It’s usually very hot and sunny compared to many other places.
So when’s the best time to visit the USA? Any time you like!