If you want to visit when tourists are at a minimum, it would seem logical to avoid winter and early spring (snowbirds) and summer (family vacationers). That leaves late spring and fall as the theoretically least crowded times.

Of course, if enough people approach the question from such a logical standpoint, they'll all go in the late spring or fall. Then, instead of beating the crowds, they'll become the crowds and the theory is shot to hell.

In other words, you can't win. Go whenever you can get the best prices on airfare and lodging.