I grew up / spent most of my pre-adult life in New York/New England, went to college in New Hampshire and spent some time in Boston afterwards.
Peak week for New England fall colors is early to mid-October, which is essentially Columbus Day for lower elevations. Taller mountains (3-4k feet) can start changing in mid-late September and reach their peak then. They might be barren by Columbus Day.
Some of the best New England areas are outside of Boston:
- I-93 up to Loon/Cannon Mt; take Kancamagus Highway to North Conway, NH, and see Wildcat/Mt. Washington, then back down to Boston
- I-89 up to White River Junction/Hanover, Woodstock, VT, and Killington. Perhaps follow Rte 100 all the way to Stowe - even go through Smugglers Notch, come back down I-89
As for Montreal, I do not know quite the optimal time, but it's similar. I believe I was up in Montreal around Canadian Thanksgiving in October, and there was still foliage due to the city being a heat sink. However, I prefer Mountains and Hills covered in fall colors. Assume you could drive out of the city to Mont Tremblant/Laurentians. Patrick?