Upvote:5
It varies from year to year and depends on the species too.
In some years it peaks mid-to-late September.
In other years it peaks in October.
Unless you live close enough to be flexible and travel on short notice, or you have enough time to travel for a long time, you'll have to gamble a bit.
It also depends on the species. I've spent Canadian Thanksgiving (2nd week of October) in the Algonquin Provincial Park, and although most trees were already barren, the yellow larches gave a very strong far-north feel within a weekend's driving distance from the Toronto area.