When is the best time of year to apply mortar in New England?
Mortar (along with all other Portland cement-based masonry products) doesn't "dry". Well, it does, but the critical point is that it cures. It absorbs and chemically combines with the water used in the mix. For this reason, it's crucial that it not 1) dry out too quickly due to solar or other heat, and 2) freeze before it's had a chance to cure over several days, at least. It also doesn't shrink or expand appreciably.

Essentially, any temperature between 32F and around 90F is fine. Outside that range and you need to protect the mortar with tarps or other means, and you may need to mist it with water to keep enough moisture present. Inside that range and you're generally good to go.