Will
I wouldn't pour concrete if it's going to drop below 20 within 24 hrs. If it is below freezing you can add concrete anti-freeze. You should be able to get this at your local hardware store or concrete plant. You also can throw insulating materials over it, blankets, hay, insulation, etc. Concrete creates heat as it sets.

Donald
Just a bit of trivia In Russia when the have to pour concrete in winter, the form work is made up from plastic that they can circulate hot water through.

Dan the brick man
mix and pour it stiff and cover it with an insulating material...it will be a while before the ground freezes that deep that you, have to worry about it...