when, on average, do the fall colors peak in Baxter State Park?

I know it's a bit early to ask about this, but I need to schedule vacation ahead of time, so I'll need the best estimate I can go with. My plan is to do a 4-day backpacking trip heading north from the main parking area to some ponds, then circling counter-clockwise and heading up over Hamlin Peak and to the summit of Katahdin, then down to Chimney Pond and then out. The daily mileage is short but I'll be lugging all my camera gear along.

I'm thinking it's probably last week of September, but I'm sure someone here would know.

Sounds about right. I did a slightly longer version of your trip a few years back during that week, spectacular and somehow quite empty of people. Once in a while it does snow a little, so check conditions out to make sure your camera stuff will be OK with whatever you encounter.

Usually I make it up for Columbus Day and foliage is a little past peak, last week of September sounds right, good luck and have fun

I would say the peak of colors is going to be the first to second week in October in Aroostook County. Usually, the colors peak in Maine around the 20th every year, but northern Maine I would hazard would be at least a week earlier. September I feel is going to be way too early for peak colors. You will see some though late September.