It depends on the area. Generally the further north, the earlier fall arrives. Go too far south and there isn't a real fall. For good fall color you want temperature where the day is still warm but the night gets chilly (the brightest colors usually follow right after a cold snap), and better in areas that have had good rainfall, as dry spells often result in leaves just falling off without much color display.
Here are some links to when different areas peak US Region autumn colour displays
If you do decide to swing by the NE to MidAtlantic region, I'd love to have you visit. In my area there's sugarloaf mountain
, Great Falls, and, if you want to go a little further North, there's Longwood Gardens
, which I think is one of the nicest places on Earth, but I grew up there so I'm biased.
Fall colours are amazing when they're at their best. My folks came here in October '76, taking a long flight from Australia (the short flights are much less popular, as they tend to end in the ocean), and then a long drive in the night to the house in rural PA. We awoke the next morning to find ourselves deep in the wood in one of the best falls ever, a sight we'd never seen before. It's just amazing.