It is too cold to pour concrete if it has been colder than 40 degrees Fahrenheit for more than 3 days. The ideal temperature range is 50 to 90 degrees.
The ground's warmth is a factor in concrete setting properly, and that is the reason for the 3 days guideline. Concrete cures more slowly in cold weather, leaving more time for it to get damaged. Finishing concrete too soon in the cold can also cause structural damage inside the concrete. If ground is frozen or ice-covered, concrete should not be poured on top of it, though there are curing accelerants and special blankets that can be used to accommodate some measure of cold-weather pouring.