Curing concrete in freezing temps
You'll definitely want to protect the surface of your concrete. I assume that the ground is not frozen. Since the curing process actually generates some heat, only the exposed layer is vulnerable. If it freezes, less water is available to the chemical reaction. This can result in a dry cure, leaving you with a weak, powdery surface. 

Cover the pour with poly sheeting and then insulated blankets, straw, or a tent with a small heater inside. You can cause similar problems by overheating, so don't do that. Seek to just maintain temperatures above freezing.