I grew up most of my pre-adult life in New York/New England, went to college in New Hampshire and spent some time in Boston afterwards.
I would say peak week for New England fall colors would be early-mid October - 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 baren 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, could go up and see Wildcat/Mt. Washington, then back down to Boston
- I-89 up to White River Junction/Hanover, over to Woodstock VT over to Killington. Perhaps follow Rte 100 all the way to Stowe - even go through Smugglers Notch, come back down I-89
- Rte 2 through the Berkshires to Williamstown
I always found foliage to be consitent and reliable. Sometimes conditions align to make some years more exceptional, but all are beautiful.
The worst conditions would be wind and heavy rain knocking the leaves off - like an early Noreaster.
As for what conditions produce the best colors - I have no idea.
I googled/AI searched, and according to the USDA Forestry Dept:
The weather conditions that occur before and during the fall season can affect the intensity and duration of fall colors. The best fall color displays are often produced by a combination of warm, sunny days, cool nights, and adequate soil moisture:
Temperature
Warm, sunny days promote the production of anthocyanins, which produce reds in maples. However, a warm period in the fall can also cause leaves to drop early before the colors have fully developed. Low temperatures above freezing can also favor anthocyanin formation, but early frost can weaken the red color. Frost can also ruin fall colors by causing leaf cells to rupture and die.
Moisture
Soil moisture plays a role in the timing and brilliance of fall colors. The best displays occur when the soil has been moist throughout the year.
Light
Rainy and overcast days can increase the intensity of fall colors. However, windy weather can knock leaves down prematurely, shortening the color display
As for Montreal, I do not know quite the optimal time, but its similar. I believe I was up in Montreal around Canadian Thanksgiving in October and there was still foliage do to the city being a heat sink. However, I prefer to Mountains/Hillls covered in fall colors. Assum you could drive out of the city to Mont Tremblant/Laurentians. Patrick?