You CAN pour concrete in winter if you take the right precautions.

More Info: Concrete that gets cold too quickly will weaken, which is not a good start to any concrete project.  If you follow certain precautions,  you may be able to successfully pour your concrete.

Why Winter Is Not Optimal for Pouring Concrete

Concrete is a mixture of water, Portland cement, and an aggregate.  As the concrete hardens a chemical reaction occurs in which the water crystals expand between the sand and gravel and bind them together.  This process requires a particular temperature range to harden and cure, which if not met, will interfere with its integrity. If the water crystals freeze it can cause cracks and spauling in the hardened concrete.

How to Pour Concrete Successfully in the Winter

In the case of an absolute emergency, there are precautions that can be taken in order to provide for a more successful outcome.  Because pouring concrete in the winter is problematic enough, it really isn't a DIY homeowner project and should be performed by a professional.  Your professional will first have to ensure that the ground upon which he's pouring concrete is not frozen.  The ingredients used in the concrete mix will likely be heated, and a faster curing cement used.  Once the cement is poured, a liquid curing compound can be sprayed on the surface and an insulated blanket placed over the concrete.