Given how large the US is, there is no one answer as to when to visit, as it very much depends on where you want to go, and what you would like to do.
For example, in January, whilst New York is cold, you can still enjoy the crisp clear days, ski, and of course still make a difference, but if you want warmer weather, New Orleans, Miami and San Diego are delightful.
December to March is great if you would like to combine skiing (in say, Utah, California, or Colorado) with giving back in New York, California or New Orleans.
New Orleans is pretty hot and sticky in July and August, but this is an excellent time to make a difference in San Francisco and New York, and enjoy incredible scenery and spot wildlife in Yosemite, Yellowstone, the Grand Canyon and Alaska.
So let us know what you want to do and we can advise when is the best time to do it.
Ideas by month (click on a month)
Start planning your luxury tailor-made trip to USA by submitting your travel plans: Plan My Trip