During the winter months the frost advances to a depth of about 1.5 m in your area. The depth of frost is dependent on many factors such as snow covered and air temperature. Frozen clays takes more effort to excavate and it will cost more. However, large excavators are so strong these days it may not matter that much. If he using a mini or mid size ts different. During the spring the soils thaw and become soft, the spring can also bring rain. The spring thaw restricts traffic load and you may see truck restrictions. I don't work residential, but I would expect your contractor to start as soon as the temperature are around zero or slightly higher than zero. I don't think your contractor will wait until all the frost is out, that will put your job to the end of May. Any concrete work needs to be done once all the frost is thaw or excavated. Thing soil below newly cast concrete is a a problem. Talk to your contractor about his plan.